Can u use a lens of film slr with a digital slr camera, e.g canon?

Generally speaking, yes. Look in the back of your DSLR manual (if you have one) and look for a table of compatibility. SOME features may be lost with some lenses, but you will still be able to use the lens.

Yes if is an AF lenses - a lense for a Canon Rebel, EOS, etc.

A lens from a T series or an A/AE won't fit.

I think Stephen M. may have meant an EF lens.

If you look on the outside of your lens barrel you'll see it say something like "Canon Lens EF 100mm 1:2"

As long as it's got the letters EF it will work on any Canon digital SLR.

The EF represents the type of lens mount, all of Canon's current lenses use this mount. You can read more about it here: http://en.wikipedia.org/wiki/Canon_EF_le...

Or view the selection on Canon's website here:
http://www.usa.canon.com/eflenses/index....

There are also third party lenses that also fit Canon's EF mount and would therefore work fine with Canon DSLRs. Some manufacturers include Sigma and Tokina.

Note, I limited my response to Canon equipment as this seems to be what you were asking. Nikon and other SLR manufacturers use different specifications and terminology.