Raw Edible Fish - Most fish that you catch can be used as food. Right-click on a fish to eat it.
Cooked Fish - Take raw fish to a cook and have them cooked. Cooked fish will heal a player more than raw fish.
Special Fish - Some special fish, such as a 22-pound catfish, can be held in your off-hand. Place them there to brag about your catch to other people. You may find other non-edible fish that are useful for Professions. Find out what they are for before selling them to a vendor. You can also catch bloated fish which can be opened to contain items.
Clams - Right-click on a clam to open it up. You will either find clam meat, which can be used in cooking recipes, or pearls! Iridescent pearls are very valuable and can be used in blacksmithing, tailoring, and engineering recipes.
Gems - Very rarely will you find gems, such as a lesser moonstone. This item can be used for other Professions.
Weapons and Armor - You may sometimes fish up weapons and armor. Usually these items (gray items) are only good for selling to the vendor for additional cash. You may, however, find some very rare and valuable equipment if you are lucky!
Humorous Items - You may fish up skulls and other items that just exist for flavor. You can sell them to the vendor or brag about your catch with your friends.
Locked Chests - You may fish up locked chests. There may be money and valuable items inside. You will need to have the chest opened by someone with the lockpicking ability. It's often good to combine with fishing so that you can open up your own chests.
Lost Trade Goods - You may find special boxes that contain items for engineering. Save the contents and sell them to an engineer.