Back of SLR lens shows the mount to the camera
for instance Nikon screws on one direction and Pentax the opposite direction
so when purchasing a lens, make sure the lens is compatible with the camera you have, makers of lenses which are not the manufacturers allow you to buy them with the mount you need, ie: Pentax, Nikon, Cannon, etc.

Although you can't see through this lens, there is an iris inside of the lens which can open as wide as the hole on the back or close small. This opening is called the F stop.

The larger the number on the lens say F22 the smaller, a larger opening is like F2
my particular lens goes from F16 to F2
so large number small opening, small number large opening.

The smaller the F stop the more depth of field you get.

Depth of field is the focus point from the camera to and beyond the subject matter.
if you use F22, everything from the camera past the subject will be in focus.
if you use say F 8, focus is lost some distance betweent the camera and subject and from infinity back toward the camera of the same distance.
Therefore if you want everything in perfect focus use the smaller opening or larger number.
You may wonder why not then just use F22
Two main reasons, sometimes you won't have enough light to expose the film and you may have to open the iris to let in more light for the exposure
second reason
If you were taking wedding photos and you want to have the background blured to bring attention to the bride and groom, you can open the lens up and create that special effect.
another reason is have you seen the photos which tend to make you believe the water on the falls or the creek is running even in a still photo?
Well there is a trick to that, first you have to shoot the photo in low light, either early morning or late evening for reduced light
Then you have to have the lens opened up farther and then set the shutter for a longer period of time so the exposure is long, thus creating the movement look, the film is actually recording the water movement because it isn't stop action on the film, so the spray and movement are being recorded over the top for a bit longer creating the movement look on the photo.
This is why an SLR allows you to do things other cameras simply won't allow you to do, it allows you to be very creative.