Well one other qualifier for Canon cameras. "Canon designers have taken steps to ensure it is physically impossible to mount EF-S lenses on EF-only cameras. This is because the increased proximity of the lens to the sensor means that on full-frame sensor or 35mm film EF cameras the lens itself would obstruct the mirror's movement and cause damage to the lens and/or camera." (Wikipedia).
Nikon uses the F-mount (since 1959).
Canon has the EF-mount (since 1987). "EF stands for "Electro-Focus": automatic focusing on EF lenses is handled by a dedicated electric motor built into the lens." (Wikipedia)
Then "In 2003, Canon introduced the
EF-S lens mount, a derivative of the EF mount that is strictly for digital EOS cameras with a 1.6x crop released after 2003. All other EOS cameras that only have an EF mount will not mount EF-S lenses." (Wikipedia). Before the EF-mount they used the FD-mount so if you are looking at old Canon lens (made before 1987) be careful.
How does this have anything to do with Kathy's situation?
The EF mount came about because in the film days mounts/lenses weren't engineered to house digital components. Canon decided to redesign the mount system and put the AF drive in the lens. This was industry leading, Nikon took until 1991 before they decided they also needed to get the AF motor out of the body and into the lens. They argued for years that Canon's system was inferior because it would drive the cost of lenses up. Ironically, Nikon lenses cost more than Canon lenses. Anyway, Nikon came up with the AF-I and AF-S lenses which mount the AF motor in the lens body just like Canon - 4 years later.
The nice thing about the EF mount is that its large enough to mount Nikon F lenses, Olympus OM, Leica R and M42 lenses with adapters.
I doubt Kathy will hit her local Swap-O-Rama looking for 30 old Canon FD lenses for her XS, so this once again is a non-issue.
The Canon Rebel can mount ALL current Canon lenses, EF-S or EF. There isn't a lens in Canon's line-up that will not work on a Rebel XS. That's not true with Nikon. With the D40 or D60 you have some lenses that will work completely including autofocus and you have others that will mount but won't autofocus. You also are very limited in the number of affordable 3rd party lenses you can buy that will have autofocus capability too. Many unknowning buyers don't realize this until after they've bought their camera and find out later they'll have to spend a mint on the most expensive Nikon lenses to have autofocus capability on their entry level camera.
Now, if Kathy were thinking about buying a $2,700 5DMkII or a $4,500 1DMkIII she might have to worry about mounting entry level lenses on her professional grade camera. Yes, EF-S lenses without modification will not work on a professional series camera. But then most professionals buy professional grade glass for their cameras so this isn't an issue.