Game type: browser-based... um.. online... avatar-driven site? Sim? Thing?
Time demand: However much you want at a chunk. Although it may suck you in for a while...
Cost: Free, but with separate "gold" (earned on the site) and "Gaia Cash" purchased with actual money, the latter of which makes it easier to get many things.
What it is:
Their avatar creator is actually the source of the icon above, which I've had for ages, but I've never played the game until fairly recently.
The game itself is a bit hard to describe, and in fact a bit odd to describe as a "game," since it's as much social site, and so on. It's one of those nebulous things that have sprung up from the web that are hard to categorize, and have grown and evolved. But whatever it is, it's fun enough.
You log in, and create an Avatar. This Avatar starts out with a few very basic starter bits of clothing, and with a bit of gold. You also have a basic starting house, and can choose from a basic starting car (sensing a theme), and have a few cheap, basic fish for your aquarium.
From there... well, what you do is pretty much up to you. You can buy just about anything with the Gold that you can earn online doing various things on the site, but some of it has to be on the marketplace, which is a player-driven auction site, so prices can be steep. That's because some things can only be bought directly from the site using Gaia Cash, which is bought with, y'know, real money (or earned through various "offers" which, y'know, are the type you really want to avoid, unless you enjoy getting lots of spam and viruses).
Your Avatar represents you around the site, on the forums and such, and can be drastically customized, though you have to of course get the items to equip on it (or change it to a different fantasy race or what have you), which goes back to the gold/gaia cash thing. The aquariums are also fun, and those and cars can be linked in your forum signatures, showing them off to everyone. There are games that you can play, including their (very beta thus far) attempt at a browser-based MMORPG, which uses (you guessed it) your Gaia Online avatar as a in-game Avatar.
You can earn little bits of gold around the site, or by selling things you find/obtain, or earn larger amounts playing the "Booty Grab" game which is linked to the aquariums, and is a lot of fun but if I start describing every sub-game in detail, I'm going to be here forever. You can also get random items each day through "daily grabs" on each sub-site of Gaia, or once a day from the "Overseer" of your aquarium, and through other special occasions such as fish dropping presents for you when they die.
I realize that this is a bit rambly, and may not give a very good idea of the site, or why it's fun and can be addicting in short spurts. That's because it's fairly hard to pin it down to just what it is... but for my part, I find it fun. There's pretty much something for a lot of different people looking for different things to do. You can concentrate on pimping out your avatar, or in your car(s), or your house, or play around with your aquarium, you can participate mostly in the social aspect, or play the mini-games for a break now and then... basically, how you play it is up to you.