For those who are not photographers, it's hard to know what the markings on a lens mean.  Here, I'll show you the basics of how to read them.

The information on the front of the lens will read something like this:

MD Rokkor-X 1:1.4  f=50mm 

In this case, MD is the mount; this lens will fit any of the manual-focus Minolta 35mm SLRs.  (This may not be present on any given brand, and probably will not be present on a third-party lens such as a Sigma or Vivitar.)

Rokkor is the brand name; in this case it indicates a Minolta lens.

1:1.4 indicates that the maximum aperture of this lens is f1.4.  This information is present on every lens I’ve ever examined.

F=50mm indicates that the focal length of this lens is fifty millimeters.  Again, this information is present on every lens I’ve ever examined, though it may be given in cm (centimeters) or, very rarely, in inches.  (To convert from cm to mm, multiply by ten; to convert from inches to mm, multiply by 25.4.)

If the lens shows f=70-210mm, this indicates that the lens is a zoom lens having a focal length which can be varied between 70mm and 210mm; this lens may also have the aperture given as 1:3.5-4.5, indicating that the maximum aperture varies between f3.5 when at 70mm and f4.5 when at 210mm.

There may also be a symbol that looks like an 0 with a slash through it, followed by =52mm.  This indicates that the diameter of the filter threads is 52mm.

In cameras using 35mm film, the following list should give an idea of the class of lens associated with the various ranges of focal length:

5mm-18mm: extreme wide-angle lenses, sometimes called fisheye lenses, though strictly speaking the fisheye lens must produce a severe “barrel” distortion; that is, it will make square objects round.

16mm-24mm: very wide-angle lenses.  These usually do not have the severe distortion; meaning that things viewed through them look relatively normal, though distant.

20mm-40mm: Wide-angle lenses.  Things viewed through these lenses should look relatively normal.

45mm-60mm: normal lenses.  These record pretty much what the eye sees.

60mm-100mm:  portrait lenses, aka mild telephoto lenses.  These produce a mildly magnified image.

100mm-200mm: telephoto lenses.  These produce a moderate magnification.

180mm or more: long telephoto lenses.  These produce a significantly magnified image; the longer specimens are telescopes.

Understand that these categories are approximate and for cameras using 35mm film only; a medium format camera such as a 120-film Hasselblad will use a lens of around 110mm as normal!