Objects can be rotated with the 
Rotate toolbar button or select from the main menu: Modify > Rotate.
The command will prompt you with the following:
Specify rotation center point <ENTER for selection center>
Pressing enter or right-clicking will cause the rotation center point to be the center of the selection. Alternatively, enter or click a specific rotation center point.
You will then be prompted with the following:
Rotation Angle (press 'R' to enter by reference):
By Reference (Press R)
Click first on a reference point, and then on the target rotation point.
By Mouse Movement
Either enter an angle in the console, on the numeric pad, or move the mouse up and down to rotate interactively.
To fine-tune the rotation amount, hold down shift while moving the mouse. This causes rotation to occur in .1 degree increments.
Note that if angular snap is enabled, then the rotation angle will snap to values on the snap increment. Even if angular snap is not turned on, holding down Ctrl will cause angular snap to take effect.
Alternatively, hold down the Alt key while rotating. This will cause the angle to be computed as though you're dragging a line around the center (imagine a clock hand). This is sometimes useful in order to snap an angle to an existing object.
See also