There are many different kinds of sea snails or marine gastropods, but their most common purpose is ridding aquaria of algae and leftover foods. A few species are also bought for decorative purposes.
Many of them can however be a nuisance in coral aquaria, but it is often difficult to identify which ones.
The information found on this page is primarily on the subject of snails found in the aquarium trade.
Pictures of many types of snails can be found here: Hardy's Internet Guide to Marine Gastropods.
Several Nerita species live above the waterline in mangrove, therefore they might crawl out of the aquarium, while other species are excellent algae eaters for marine aquariums.