Well... Is this a tough question or what?
I guess Disco means different things to different people. Some people will think of their youth and laught about all silly things they did. Others would say it was the worst type of music of music they ever knew… No matter what you think of Disco it seems like everybody have got an opinion about it.
I'll give you mine about Disco...

Disco music is really my life, maybe because I was young when Disco hit the dancefloors and that I was brought up with it. I used to listen to this great music coming out of my radio and I taped like crazy. (Unfortunatly I was a bit too young to go to the clubs and hear these great songs the way they are supposed to be heard... Loud with a lots of bass that almost punches you in the stomach at every bass bump! Later as a DJ I got the chance to pump these great Disco songs the right way to my audience.)
To me Disco stands for a happy music specially made for dancing to. Because this was really the purpose with Disco. Get people on the dancefloor and make them dance!
Or as Joey Negro (famous UK remixer) once said; "Disco music was the first music that was engineered specifically for clubs - it was made with the dancefloors in mind."

Beside Disco's dancefloor potential was it also a very alive, chearful and positive music style. Just by hearing the titles on some of the Disco songs you understand what I mean... "Good Times" by Chic, "Born to be alive" by Patrick Hernandez and "Strutt your funky stuff" by Frantique.
Other songs full of life and energy is for example "Copacobana" by Barry Manilow, the Village People's classic "Y.M.C.A." and of course one of my very favorites (which my web-page is named after) "Disco Inferno" by Trammps.
Sometimes Disco was very experimental and futuristic, like for example the Giorgio Moroder penned Donna Summer anthem – "I feel love". This song together with the Star Wars movies made the way for "Space songs" like the Nile Rodgers & Bernard Edwards written "Spacer" performed by the french Shelia B. Devotion. But the queen of Space must be Sarah Brightman (yes that's the same lady that's hot right now) who sung for example "I lost my heart to a Starship Trooper". And, of course, MECO's Disco version of the "Star Wars" theme.
The Space was also a very popular subject in the Funk scene. Since Funk, Soul and R'n'B is very related to Disco I have to mention Funk here as well. The influences between the different styles makes it sometimes very hard to put a specific style to some songs. The "Space Guru" himself for the Funk scene is (of course) George Clinton. He had his Mothership hanging over several P-Funk acts like Parliament and Funkadelic.
Later Space-funkers are for example Zapp with their space-like sounding Roger who used to use a vocoder to get his special voice.

The big Disco labels all had "Their" sound. Often depending on where they were based. Salsoul was a New York based label often with big Orchestral arrangements and almost a symphonic type of Disco. Other New York based labels were for example Prelude and West End Records. Prelude made a little more uptempo Disco and West End on the other hand made a slower type of Disco.
The BIG "mainstream" labels like the also New York based Atlantic took on the bigger and more famous acts. Atlantic had for example acts like Chic, Sister Sledge, Trammps, Boney M and ABBA in their stable.
On the West coast the BIG Disco label was the Los Angeles based Casablanca with mega stars like Donna Summer, Giorgio Moroder, Kiss and Village People.

So... I guess I haven't been able to tell you what Disco is… This was not the purpose either - But I sure hope I managed to give you some more ideas of how different types or styles of Disco could sound like.

Take care and Disco-on,