If you're wondering what a focal length is, there's a really technical explanation here. I have no idea what all the jargon means. :) But in simple terms, the focal length of your lens affects how magnified everything looks when you look through the viewfinder. A longer focal length like the 85mm will make everything look much closer to you than a shorter focal length like the 35mm. The three lenses I own don't zoom. They are prime lenses so if I need to "zoom in", I need to physically move closer to what I'm photographing.
For my highly technical experiment, I grabbed the cutest, non-mobile subject I could find (Timmy), stood in the same spot that I marked with a dog toy, and took a picture of him with each lens. For each picture, you'll notice changes in two areas: how much of Timmy and his surroundings are in view and the amount of bokeh (background blur) in each photo. Aperture is not the only thing that affects bokeh. Your focal length does too! The longer the focal length (so the more "zoomed in" you are), the more blur you will have. (Note: I kept my aperture the same for each photo so that the only thing affecting bokeh was the focal length.)
~ 35mm ~
35mm, f/2.8, 1/250, ISO 100
~ 50mm ~
50mm, f/2.8, 1/250, ISO 100
~ 85mm ~
85mm, f/2.8, 1/320, ISO 100
As you can see, the longer the focal length, the more zoomed in everything looks even though I stood in the same spot for each picture. Also, the bokeh got more and more blurry as the focal length increased.
If you're wondering what my favorite lens to shoot with is, the 35mm wins for indoor use. The 35mm is SO sharp even at 1.8. I find that I generally don't have enough room to shoot with the 50mm and the 85mm in my house. For outside, the 50mm is my go-to lens for almost everything, but I love the 85mm for a nice portrait shot (for people who can actually stay still unlike my wiggly kiddos!).
What is your favorite focal length to shoot with?