Numbers followed by mm followed by f/# is describing a lens
ex.
70-200mm f/4L is USM
70-200 means it has a "zoom range" of 70mm which is a wider angle of view to 200 which is very tight or zoomed in.
f/4 means that the aperture of the lens opens up to 4.0 at its widest, that's the most amount of light that lens can let in.
is and USM are canons terms meaning Image Stablizer and Ultra Sonic Motor autofocus.
A lens that only has one number ie. instead of 17-50 f/2.8 it's just 50 2.8 means it's a prime or fixed focal length (another way to say the amount it zooms) lens. These lenses have their advantages however they don't zoom which is why there's only one number.
Here's an image to help you figure out what lens lengths mean.
Cameras are usually described with letters following numbers (With MANY exceptions)
Example TM700, CX100, FS100.
Canons DSLRs are described with a number followed by D meaning digital.
Example Canon 60d, 5d mkii (meaning it was the second version), 7d, 550d
Nikons are the opposite
D7000, D40, D4x
This is just grazing the surface but I hope it got you started in understanding.
