There are three main types of marine aquarium:

1) Fish Only - marine aquaria that house only fish, no corals or inverts (live rock may or may not be used)

2) Reef - marine aquaria containing live rock, corals, inverts and fish (but doesn't have to contain fish)

3) Nano Reef - 'mini' reef tanks, anything from 5 gallons to 30, considered to be a 'nano', anything smaller is a 'pico' reef!

The first two types are covered in this forum section... Nanos have their own section here...