What lenses can I use with my Nikon D40 SLR?

I have just bought my friends SLR Camera which comes with default lens but would like to get a telephoto lens too. I have seen some Canon ones, inparticular a Canon USM model (which I understand means the motor in built in) but are Canon lenses compatible? If not are there any makes, apart from Nikon's own stuff, that I should be looking out for?

Definitely not Canon! You can use Nikon lenses, or lenses by independent manufacturers, the best are Tamron, Sigma, and Tokina, but you must buy in Nikon fitting. The D40 does not have a focus motor built in, so you need to buy lenses with focus motors built-in - there's a good selection.

Best Nikon options would be the 55-200 VR and the new 55-300. Sigma also has an alternative offering in this range but it is not that much cheaper and the nikons are fine quality.

55-200: http://www.bhphotovideo.com/...ikkor.html which costs about the same as the canon.

55-300: http://www.bhphotovideo.com/...4291315846

Thom Hogan has recently reviewed the sigma: http://www.bythom.com/Sigma55-200lensreview.htm and concludes that the Nikon is a much better option.