Here's a handy reference guide showing the differences between a wall cloud and a shelf cloud.

WALL CLOUD vs. SHELF CLOUD:
A WALL CLOUD is associated with the UPDRAFT portion of the Thunderstorm.
A SHELF CLOUD is associated with the DOWNDRAFT portion of the Thunderstorm.

A WALL CLOUD slopes TOWARD the rain area.
A SHELF CLOUD slopes AWAY from the rain area.

A WALL CLOUD is most often associated with rotation and tornadoes.
A SHELF CLOUD is most often associated with straight-line winds.