Scientific Name: 
Family:               Porifera
Common Name: 

Description:
Sponges are stationary filter feeding organisms which exist in all reef tanks.  They can be dull gray or light brown colored blobs or they can be attractive in both color and shape.  Yellow and purple colors are common.  Sponges will normally have one or more openings through which water passes and is filtered for its food value.

Good or Bad?:
Sponges are mostly innocuous.  Most are smallish and hidden from view until a piece of live rock is moved those exposing them.  Sponges do help to clean the water and are a good sign of a overall healthy tank.

Notes: 
Sponges are not photosynthetic and must catch their food to survive.  Tanks will low protein skimming levels and those which are feed small foods such as phytoplankton will tend to have more sponge growth.  Sponges tend to inhabit areas of the tank with low light levels.

Further Reading:

Sponges — pump-filter modules through space and time…
By Ron Shimek