As I mentioned before, you are playing through this bullet riddled planet of Pandora, killing those that get in your way in constant search of the Vault. But the great thing about Borderlands gameplay is that it's actually better with a friend. You can play Borderlands as a two-player, or four-player mode - and yes, you can still solo it. The game is actually designed for multi-player cooperation, and it helps to go through the game easier and faster.
But, for those that just want to know about the Borderlands gameplay in general - I have to say it's downright awesome. While it's a FPS, hack and slash, you name it - the bloody gore of whacking some great mobs as you go with tons of great, useful weapons is unbeatable. There are literally hundreds of different weapons that you'll find in Borderlands, along with shields and other great perks to help you along.
While you go along, killing your way toward the Vault, you'll gather up loot and be able to use it along the way. This is great, as you can get some great stuff with it.
You'll travel through Borderlands the game, smashing and crashing, killing and looting, causing all types of problems and causing major mahem along the way - all while you're sitting on your couch grinning like a kid in a candy store. The Borderlands gameplay will provide you with hours on end of gory fun that cannot be likened to anything else out there.