I was chatting to someone yesterday who, it turned out, had never heard of tower defence games. Admittedly it's a strange little subgenre. What's most strange about it is that despite being one of the more OCD elements of the real-time-strategy milieu it seems to have become largely the province of "casual gamers".
The premise is that a bunch of enemies will troop across a screen from point A to point B, and it's your job to install a bunch of automatically firing turrets along the way to wipe out the nasties before they reach their destination. Typically the enemies will come in waves; you'll have limited resources to build your towers, but every enemy killed or wave cleared will grant bonus cash.
As far as I can tell the first instance of tower defence gameplay appears in Atari's classic Warlords (1980), although it's more properly inspired by Westwood's Dune II (1992), where correct alignment of your turrets and tanks was key to success.
Anyway, to educate the uninformed I thought I'd trot out three of my favourite browser-based tower defence games, and let the gameplay therein say more than the written word ever could.
Desktop Tower Defence 1.5: The undisputed king of browser-based tower defence. Clean, engaging graphics, a good mixture of depth and intuitiveness, and a wide range of modes to try.
Vector TD 2: Deeper than Desktop, with more turret options, and with more freedom to set your own pace. Simple vector graphics emphasise the tactical element of the game.
Bloons TD 2: Monkey versus balloons in a kid-friendly grudge match. Turrets here don't snap to a grid, which can throw off those who like an orderly turret layout, but a lot of charm and a wealth of turret options compensate.
Try them out, but be warned - these are very addictive games!