Fish are divided into two main groups, jawed and jawless. The jawed group can be broken down further into two groups according to the composition of their skeletons. Fish that have a skeleton composed mostly of bone, are referred to as bony fish. The other group of fish have a skeleton made up of mostly cartilage.

The following is a general overview of the external anatomy of bony fish.

Body shape
Most fish have a streamlined shape. Some of the fast swimmer, such as Tuna, have a torpedo shaped body. Other fish, such as flounder, are flat which enables them to lay on the bottom undetected. Some fish look like rocks and others resemble weeds. This is referred to as protective resemblance, which helps a fish escape its enemies or go unnoticed by its prey.

Scales
Typically, most jawed fish are covered with scales. These scales grow on the outside of the skin and overlap like shingles on a roof. Numerous glands on the epidermis secrete a mucus, often referred to as slime, which protects the fish from bacteria and harmful parasites. The mucus also reduces friction as the fish travels through the water. You can usually tell the age of a fish by looking at its scales. Similar to the rings of a tree, the spaces on the scales can provide a record of growth-rate and age.

Fins
The fins on a fish are used for the purpose of movement and balance. Fins can also be seperated into two categories, median or paired. Median fins are single, verticle fins are the back and underside of the fish. Paired fins are identical on each side of the body.

Median

The dorsal fins are located on the back of the fish. Most fish have at least one which is used to keep the fish upright.

The anal fins, which are located on the underside near the tail, are also used to help keep the fish upright.

The caudal fin is at the end of the fish. This fin is used for thrust and/or propulsion through the water. It can also be used for steering.

Paired

The pectoral fins are located on the side of the fish, just past the back of the head. These fins are used for stearing, stopping and maintaining position.

The pelvic fins are located below the pectoral fin and are also used for stearing, stopping and maintaining position.