Reference angles are always acute (between 0 and 90 degrees, or between 0 and pi/2 radians) and are formed between the terminal side of an angle and the horizontal axis. Here is a picture of what a terminal side is:

The red line in the images is the terminal side, while the blue is the horizontal axis, or x-axis.

If a standard angle, theta, is in Quadrant I, then the reference angle is also theta degrees

If a standard angle, theta, is in Quadrant II, then the reference angle is 180-(theta) degrees

If a standard angle, theta, is in Quadrant III, then the reference angle is (theta)-180 degrees

If a standard angle, theta, is in Quadrant IV, then the reference angle is 360-(theta) degrees

Example: 288 degrees would have a reference angle at 360-288= 72 degrees

Think of the reference angle as this: the acute angle formed when the standard angle sticks to it's closest x-axis area (the 180 degree line for Quadrant II and III and the 360 degree line for Quadrant I and IV).