There are two basic kinds of traps: structural and stratigraphic. Structural traps are the result of deformations of the rock layer. Examples of structural traps are anticlines and fault traps. The fault trap is associated with the shifting of fault layers along a fault line, something that we are familiar with as the cause of earthquakes if the shifting motion is strong enough. Stratigraphic traps form when reservoir rock is cut off by a horizontal layer of impermeable rock. The figure below shows oil pooling in the two different types of structural traps. The dome-like structure on the right is an anticline, while the structure on the left is a trap formed along a fault.

Information provided by: http://www.unb.ca