The x,y,z parameters to glRotate* describe a unit vector from the origin (it'll be normalized if it isn't already) that describes an axis to rotate around. In other words, if you have a vector of (1,0,0), it'll rotate around the X axis. If you use (0,1,0) it'll rotate around the y axis, and so on for the z axis. A vector of (1,1,1) will rotate around an axis that's 45-degree angle from all of the axes.

The rotation follows the right-hand rule, meaning as you look down the vector to the origin, the direction of rotation is counterclockwise. It's called the right handed rule because if you hold out your right hand and point your thumb in the direction of the vector, the rotation will follow the direction in which your fingers curl.