What does that f-number mean in camera specs?

[bdawg923]

When I see a camera on a website, under the specs (or sometimes in the name) it says something like [this is from [ame="http://www.amazon.com/Canon-30D-Digital-18-55mm-3-5-5-6/dp/B000DZFPKC/ref=pd_bbs_sr_3/103-0553374-8138221?ie=UTF8&s=electronics&qid=1180205216&sr=8-3"]here[/ame]] "Canon EOS 30D 8.2MP Digital SLR Camera Kit with EF-S 18-55mm f/3.5-5.6 Lens". What does 18-55mm mean and what does that f/number mean?

If possible, could give give examples of what happens when those numbers are changed (eg: if the mm increases, the lens is finer and probably more expensive [that's just a guess], or if the f/number is decreased the image is brighter or whatever)?

Secondly, when I see a camera on amazon.com for example, some of them say (body only). What does that mean? [ame="http://www.amazon.com/gp/product/B000DZDTKU/sr/ref=pd_cp_e_0/103-0553374-8138221?ie=UTF8&qid=1180205216&sr=8-3&pf_rd_m=ATVPDKIKX0DER&pf_rd_s=center-41&pf_rd_r=0F69DZFBMF7H08HHH8CZ&pf_rd_t=201&pf_rd_p=250314001&pf_rd_i=B000DZFPKC"]example[/ame]

Thank you.

 

[EOS_JD]

Read these

http://www.luminous-landscape.com/tutorials/focal_lengths.shtml

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

http://www.paragon-press.com/lens/lenchart.htm

http://www.photoaxe.com/understanding-the-lens-focal-length-and-aperture/

 

[JIP]

The 18-55 number is the focal length of the lens ex. 18 means a very wide lens and 200 would be a long zoom lens so it would bring things in closer. The F number is the maximum aperture of the lens and a measurement of the lenses light gathering capabilities and is a good way to judge the quality of the lens meaning a lens with a max. aperture of 2.8 is going to be better than the lens you listed. And yes "body only" means no lens.