There are three main types of automatic vacuums used on inground pools today.
Pressure Side Vacuums - These vacuums are self contained and rely on the pressure of the pool pump to propel the vacuum around the pool. They do not require a separate filter to work properly. The debris is trapped in a separate filter bag. This is the most common type of vacuum used on a fiberglass pool.
Self Contained Robotic Vacuums - These vacuums offer the best performance of any vacuum. With the performance comes a much higher price tag. These vacuums are self contained and do a good job of vacuuming the entire pool.
Suction Side Vacuums - These vacuums use the suction line from your pump to move around the pool. Because fiberglass pools have a smooth surface, this type of vacuum does not work well. Another reason you do not need this type of vacuum is the fact that fiberglass pools do not need to be scrubbed to remove debris from the surface.