A prime lens is a lens in which the field of view (focal length) cannot be adjusted. The only way to take in more or less of the scene is to walk closer to or further from the subject. (sometimes called “zoom with your feet,” though this is technically inaccurate since changing camera position is not the same thing as adjusting the focal length) Or you have to carry a selection of lenses of different focal lengths with you and swap them out as required.

A zoom lens is a lens in which the field of view can be adjusted. If you can’t fit in all your friends in the picture, for example, you could just rotate the zoom ring on the lens until they’re all in there. Or if that bird is too far away you could rotate it the other way to zoom in closer.

Up until the late 1980s prime lenses were the most common lens sold, because from an optical design standpoint it’s much easier to design a high quality prime lens that can take nice sharp photographs than a decent quality zoom lens. But a prime is also obviously a lot less convenient, since you have to move around more to fit stuff in. So by the late 80s zoom lenses became more and more popular. Today hardly any low-cost lenses are prime lenses, because everybody wants zooms.

So why buy a prime lens at all? Well, for the same reason as ever - prime lenses are easier to build and offer fewer compromises in design. If you want a really sharp crisp lens, then a good prime will offer sharper pictures than most zooms. Or if you want a lens that can let in lots of light and thus can be used for low-light photography then you’ll probably want a prime, since it’s much harder to build a “fast” (lets in lots of light) zoom lens. And some crusty old photographers also argue that using prime lenses is very important for novice photographers since it forces them to learn about the importance of focal lengths and perspective.

Lens construction is thus always about tradeoffs. You may want a lens that’s small, lightweight, has zoom capabilities, lets in lots of light, is really sharp, has high contrast, doesn’t distort the image and is cheap. But in real life you can only get some of those properties - it’s impossible to get all of them, sadly.

Most amateurs on a budget choose the flexibility of low-cost zoom lenses over picture quality as their compromise. Many advanced amateurs choose the higher picture quality of affordable primes and deal with the inconvenience as theirs. And many professionals buy high quality zooms that weigh a ton and cost piles of money as theirs.

Note one common misconception - a lot of people think zoom lenses are used for taking photos of things far away. That’s actually a better description of a telephoto lens, as described above.