F# in lense description...

[Gandalf]

Hey everyone, this is a fairly simple question, but one that has been bugging me for awhile

What exactly does the F/# mean in the description of camera lenses?

Take this one for example:
http://www.adorama.com/CA702004AFU.html

It says Canon EF 70-200mm f/4L; some say f/5.6, some say f/4-5.6, but what exactly does that mean?

What # is more desirable?

 

[SWFLA1]

Take a look here... hope this helps...

http://www.uscoles.com/fstop.htm

 

[Gandalf]

Take a look here... hope this helps...

http://www.uscoles.com/fstop.htm

Ah ok, thanks for the link, I think I understand the number now!