A genre of computer games, falling directly on the line between 3rd-person action games (like Diablo, Max Payne and Metal Gear Solid) and real-time strategy games (like Starcraft, Age of Empires and Total Annihilation).
Having its roots in the classic sci-fi thriller X-COM: UFO Defense, and its gold standard in Jagged Alliance, war games involving squad-based tactical combat are perennially popular. Recent titles in the genre include: Freedom Force, MechCommander, Star Trek: Away Team, Rainbow Six and Fallout: Tactics. Also, there's been a recent wave of "tactical RPGs", role-playing games that let you control all the characters in your adventuring party. For instance, the Infinity Engine games-- and especially Icewind Dale-- could arguably fall into the 'squad-based tactical' category, as well.
As the name suggests, these titles feature a style of play that is more fast-paced and visceral than an RTS, yet more in-depth and challenging than your basic first-person shooter. Instead of a one man army, the player is usually in control of between 4 and 8 characters, all with their own skills, weapons and weaknesses, which you have to use as an efficient team in order to accomplish your objectives.
Typically, the main challenge of these games is having to come up with new tactics on-the-fly, in order to combat enemy forces that become increasingly well-staffed, well-armed and well-positioned as the game goes on. Instead of building new facilities and advancing through a tech tree, as you would in a strategy game, new weapons and equipment are usually acquired through battlefield salvage, or purchased between missions from 3rd-party dealers.
The interface for this type of game is usually quite simple, and will also be almost identical from game to game. Click on a character to get his or her attention, or click and drag to give orders to a whole group. Click on a empty piece of open ground, and the character will move there. Click on an enemy, and characters will attack it. Click on a destructible building, and it'll be destroyed. Click on a friendly unit or structure to interact with it. You get the idea. There'll probably also be quick access keyboard commands you can use to do all the same things. Along the bottom or side of the screen will be status icons to represent your team, with meters that keep track of each characters' health, ammo and-- depending on the setting-- armor, magic level, fatigue, etc. All of the newer games also have a radar/mini-map feature tucked into one of the corners.
If you like real-time strategy games, but tend to enjoy micromanaging small, elite fighting units instead of marshalling vast faceless armies-- or if you like action games, but wonder what Metal Gear would be like with an entire team of operatives instead of just Snake-- then maybe you should give squad-based tactical games a try.