In Limit, assuming you don't check, you must bet exactly the amounts prescribed by the game in each round — no more and no less.
Limit is always described by two numbers: The biggest is twice the size of the smallest — for example, $10/$20 (said, "ten-twenty" with the $ ignored). The smaller of the two numbers is the exact amount players must bet (or raise) after seeing their hole cards and the flop. This means the smaller number is also the size of the big blind (because that is the forced first bet in Hold'em) and the small blind is half that amount ($5 in a $10/$20 Limit game). The larger number is what must be bet (or raised) on the turn and the river.
In casinos and cardrooms, Limit games typically start at $2/$4 and run up into the hundreds of dollars. Limit typically has a minimum buy-in of ten times the smaller bet size. So a $2/$4 game requires a buy-in of at least $20. There is no maximum.
Limit tends to be the hardest variation of Poker to bluff (bet as though you have a good hand when you actually don't in an effort to get others to fold) because the bet sizes are regulated.