Nikon haven't changed the mount since 1959, Canon changed it 1987 - every EF / EF-S lens from then on had autofocus. Old FD lenses can't be put on EOS cameras.
Nikon just changed some things so their motor was sitting in the body first and now they are building more and more lenses with motor in their lenses.
In short it's more complicated with Nikon. Canon just went from FD (manual focus) to a new mount (EF) so any lens with EF fits any Canon EOS and will autofocus if it's not broken
EF-S lenses are similar to EF lenses but they will only fit crop frame cameras. Crop frame cameras are the payable ones
p.s. - what is a good entry level flash to purchase for the T3i, Speedlight I think they are called? Something with bounce capability please.
If I had to choose something that's not too expensive I'd go for the Nissin Speedlite Di622. Even if you decide to upgrade later on, you can use it as a second flash (slave), which can be pretty handy.