Bolos are awesomely powerful self aware robotic fighting vehicles designed to defend Humanity from all aggressors, sometimes even itself. Similar in concept to an OGRE, Bolos differ in that they always carry a human commander.
Bolos are the creative invention of the late Keith Laumer, and there are many books written about them and their military exploits. The stories (by various authors) are brilliant. If you like OGREs, you'll LOVE Bolos!
To learn everything there is to know about Bolos, check out Bolo Central Command ; the most comprehensive Bolo data archive around.