Disclaimer: There is a wide diversity within each of the streams of Judaism. A lot of what I will say are generalizations and may not apply to every community.

1. Size of the community: In general, Orthodox synagogues cater to a population walking distance to the synagogue. Conservative and Reform synagogues often cater to a larger area. Therefore, they are often larger, with larger buildings and more family membership units. (Of course there are some large Orthodox synagogues, and small Reform communities, but I'm speaking in generalizations here).
2. Singing in the service: It is pretty common for Conservative and Reform synagogues to have a professional cantor--i.e. a professional who has an advanced degree in Jewish music to lead the singing in the congregation. Lay-led services are more common in Orthodox synagogues (and thus the quality of the singing isn't generally at a professional level). Conservative and Reform services generally have more singing than Orthodox services. Choirs are not uncommon in Reform and Conservative communities, but are not common in Orthodox communities outside a few specific communities.
3. Music in the service: Orthodox services generally are without musical instruments, especially on Shabbat. Conservative services occasionally do include musical instruments, but many do not. Reform often includes musical instruments. Reform communities tend to use more contemporary melodies and musical styles, while Conservative and Orthodox communities tend to use more traditional nusach (liturgical melodies), though Conservative services sometimes include some innovative melodies as well.
4. Observance of the community: As others have said, in Reform community, the stress is on creating your own meaningful religious practice and less on strict observance of ritual law. Conservative communities, while still holding on to Jewish law and aspiring towards full ritual and ethical observance of halacha, you'll find that there's a wide diversity of levels of observance, from "Conservadox" (i.e. pretty much Orthodox in all but name) to not observant at all, and everything in between. Orthodox communities generally expect full observance of its congregants; though of course not all congregants at every Orthodox community is fully observant.
5. Gender: Reform communities and most (but not all) Conservative communities are egalitarian; that is women and men sit together, they participate equally in the service, they count in a minyan, they may serve as rabbis, etc. There are a few Conservative communities that are not fully egalitarian, whether by incorporating separate seating, or not counting women in the minyan, or not letting women read from the Torah, but these are very few nowadays, and mostly in Canada (I believe). In all Orthodox communities, there are separate roles for men and women. While the left-wing most fringe of Orthodoxy have started allowing women to take on clerical roles, most of Orthodox do not accept women as rabbis. Orthodox synagogues generally have a barrier separating men and women, and most do not give women synagogue honors or leading roles in the service. There are some left wing minyanim called partnership minyanim that maintain the barrier separating men and women, but allow women to take some honors and leading roles in the service--these minyanim generally don't identify as either Conservative or Orthodox.
6. Microphones: Orthodox communities by and large do not use microphones on Shabbat (although some used to). Reform and Conservative synagogues generally use microphones on Shabbat if needed.
7. Decorum: Orthodox communities often have less of an expectation of decorum than their more liberal counterparts. It's common in Orthodox communities for people to walk in late and catch up with the davening at their own pace (so some people may be standing while others are sitting). People do not necessarily recite prayers in unison, instead saying them (or mumbling them) at their own pace (the exception is in Sepharadic synagogues, which are much more in unison with the hazzan/prayer leader). It's common to have children (who are not legally bound to regular prayer) to run around and play games. Reform services often took a page from Christian expectations of behavior in church--kids are often expected to sit quietly and follow along, people are expected to show up on time, people are expected to sing and recite prayers in unison (not at their own pace), and chatter during the service is not acceptable. Conservative synagogues tend to be somewhere in between. At Conservative synagogues, it's acceptable to show up late, through fewer people are trying to catch up and do earlier parts of the service mismatched with the rest of the congregation. Often Conservative synagogues have "children's services" for kids. Generally prayers are done in unison, with substantial singing.
8. Emphasized services: In Reform synagogues, often the Friday night service is the main service of the week, with Saturday morning services being smaller affairs. Sometimes they even include Torah readings on Friday night. In Conservative and Orthodox synagogues, Saturday morning services are generally the largest of the week, and Friday night services are generally relatively short and definitely without Torah reading. Of course most Orthodox synagogues and some Conservative synagogues offer mincha Shabbat services as well as weekday minyanim as well.
9. Liturgy: The core of the liturgy will be very similar amongst the movements: Introductory blessings, the Shema, and the Amidah. However, Reform synagogues often change some of the wording (for example in the Amidah they say "who gives life to all" instead of "gives life to the Dead) and remove large passages that don't fit their ideology. The emphasis is often on singing songs rather than reciting long passages of prayer. Reform synagogues will often include substantial content in the vernacular (e.g. English), and responsive readings are quite common. Orthodox services generally include full liturgy, with very little flexibility on changing or skipping parts. Furthermore, Orthodox services tend to be entirely in Hebrew (including Prayer for the country, etc). Conservative synagogues are somewhere in between. They generally maintain traditional liturgy, while making a few changes to remove objectionable content (like "she lo asani isha"). Conservative services are mostly done in Hebrew, but the prayer for the country is often done in the vernacular, and an occassional Engish responsive reading here and there is not unheard of. They may condense or skip certain introductory or concluding content, and may include some alternative prayers here and there.
10. Torah readings: Reform syangogues generally follow the trienniel (3 year) Torah reading cycle. Most Conservative synagogues do as well (though some don't). Orthodox synagogues follow the 1 year Torah reading cycle.
11. Talitot and kippot: In Reform synagogues, often a talit and kippah are optional for both men and women. In Conservative synagogues, a kippah is required for all men and boys and a talit is expected for Jewish men above Bar Mitzvah age, while the kippah and talit are optional for women. In Orthodox synagogues, the kippah is required for boys and men (at all times, not only in the synagogue), but the talit depends on the community (in many Orthodox Ashkenazi communities, they only start wearing a talit after getting married, while in Sephardic communities they often start wearing the talit at Bar Mitzvah age). Women in Orthodox communities generally are forbidden from wearing a kippah and tallit, through married women are often encouraged to cover their hair either with a scarf or a wig.
12. Marriage: Reform rabbis will often (not always) perform intermarriages and generally perform same-sex weddings. Conservative rabbis will not preform intermarriages, and some but not others will perform same-sex marriages (between two Jews). Reform and some Conservative rabbis will perform marriages between a kohen and a divorcee or a convert. Orthodox rabbis will not perform intermarriages, same-sex weddings, or marriages between a kohen and a divorcee or convert.
13. Halacha: Orthodox communities tend to view halacha as binding. They generally say you should follow majority opinions in the Talmud, and they view later law codes (like the Shulchan Aruch) as authoritative. The Conservative movement is also a halachic movement. However, they are more open to considering minority opinions, adopting opinions different to what is found in law codes like the Shulchan Aruch, taking into consideration conflicts between different halachic principles, and using takkanot (halachic decrees) to modify existing halacha if a need arises. Reform communities generally believe that halacha should be the basis of our ethics and morals, but Jewish law, especially in regards to ritual, is not longer binding. Instead, people should be free to develop a ritual practice that is meaningful to them.
14. Accpetance of modern scholarship: Reform and Conservative is generally open to academic scholarship of the Bible and Jewish tradition. A wide range of views on theology as well as Biblical authorship and historicity is accepted. Traditional Torah study and academic approaches are used side by side. Orthodoxy tends to be skeptical of academic scholarship of the Tanach, favoring the methods of traditional Torah study instead. While there can be some differences in views on Biblical interpretation accepted within Orthodoxy, generally Mosaic authorhip and the divine origins of the Torah as well as the the overall historicity of the Biblical narrative are universally accepted.