Focal length is the distance calculated in milli meter, from the center of the lens to the focusing points on the sensor plane when the subject at infinity is in focus.
A lens could could have a variable focal length (70-200) or it could be a prime lens (50mm)with single focal length.
All lenses of the same focal length give images of the same size. Image size is in proportion to the focal length, so changing from 50mm to a 100mm lens, for eg, will double the image size.
Common names for lenses from 35mm cameras:
12-20 mm Ultra wide angle lens
21-24 mm Super wide angle lens
28-35 mm Wide angle lens
35-70 mm standard lens
70-105 mm shot tele photo or portrait lens
135-200 mm telephoto or long focus lens
300 mm > super telephoto