Battletech is a game of armored combat. It can be small unit tactics or large scale strategy. It all depends on the players involved, and just how in depth you wish to get.
Units available include:
Infantry
Scout cars
Hospital trucks
Command vans
Tanks
Mechs
Submarines
Wing in Ground Effect craft
Aero fighters
Aerospace Fighters
Dropships
Jumpships
Warships
As you can see, it is quite a diverse list. And it's only partial. There are at least four or five other unit types to be had.
The game is played on a table top, or, as the largest game I ever played was doen, on the floor of a very large room. Usually the games are over in a few hours, maybe a day. The biggest one I played took two and a half months, involved at least three fist fights among the players, and got me some of my first miniatures for free (the rules we had established were, if you took them, you kept them, if you lost them... you really lost them. These are not normal rules.)
Any way, you play with 2D6, a quick reference card for roll tables, and a tape measure for movement and shooting distances if you elect not to use the standard hex printed maps. Terrain that blocks line of sight between the models on the table blocks the possibility of direct fire. Being partially concealed behind terrain grants a cover bonus. Many things factor into each phase of the game and directly effect the other phases. Movement direction and speed, heat generation, ammo capacity, weapons' ranges, piloting and gunnery skill, armor, damage, terrain, and more.
The game is loads of fun, and among a few friends, with a few beers and pretzels, is a great way to waste a weekend.