In AutoCAD, the [Enter] key and the space bar, for most purposes, are identical in function. (For example, typing LINE followed by Enter is equivalent to typing L followed by the space bar.)
Typing the space bar at the Command prompt after a command is completed will repeat the last command typed.
Click the [esc] (escape) key two or three times in succession will cancel any command and return to the Command prompt.
Each of the function keys has a special function in AutoCAD.