Please read the section on how lenses work first. LINK here to this section.

There are two basic categories of camera lenses:

Prime lenses

Primes have a fixed lens focal length, making them faster and sharper. While prime lenses are less flexible due to the fixed focal length, they are also fast and lightweight and have been designed for a fixed length so are of higher optical performance at that depth, making them easier to work with and of a higher quality for any fixed length.

Zoom lenses

Zooms use a series of lenses to allow different focal lengths from a single lens, making them more flexible but not as fast and slightly effecting optical quality at any fixed length. They contain more glass, which aids in their flexibility, but they also tend to be bigger and heavier than prime lenses.

Within both prime and zoom types of lenses, there are a variety of lenses, all with different focal lengths.