Gameplay is exactly the same as in the awesome Quake III Arena, with the same weapons, power-ups, scenery elements, etc. So if you ever played that game back in the day (or are still playing it now, since it continues to be an excellent shooter), you'll feel right at home playing Quake Live.
What really makes this online version stand out is how smoothly it runs. Of course, before you start playing you'll need to download a good amount of content, but all of the action takes place right in your browser.
Quake Live is a really fun game that will steal dozens of hours from you thanks to its rapid gameplay and frenetic action. It's one of the best out there in terms of free games.
Although you'll play in your browser, the game downloads a few hundred megabytes in game content.