When you go out to buy a camera, all these mm numbers are listed - 18mm, 50mm, 100mm, 18-50mm, etc - but what do those numbers mean?
Well, those mm numbers are the "focal length" of the camera lens. What those mm numbers actually mean is a somewhat complicated mathematical concept involving measurements and angles, but for the purpose of this page, lets just reference the mm of focal length as the distance a lens can zoom you in to. As you can see from the chart below, lower numbers are closer to you, while the higher numbers get farther away. Notice the human eye on the 50mm mark? Well, the low numbers below 50mm actually give your camera a wider view than your own human eye, while everything above 50mm makes objects seem closer when you look into the camera (this is how binoculars work). Take a look at the chart below...
As you can see, each zoom range on the chart has a recommended use. Buildings and landscapes look great when you use a 10-35mm focal length lens because it's so vast and expansive compared to the human eye. On the other end, 200mm, 300mm, and even 1200mm lenses are available to get you close to sports and wildlife that you can't physically get yourself closer to!
I hope this helps explain it, and if you have any questions or comments, please feel free to ask in the comments section below!