I'll answer some of your questions and I don't want to come off as a jerk but seriously, do some searching yourself. We're here to help, notspoon feed you. I've indicated by answers in red below.
Q: What is the difference between the Canon Rebel xt, xti, and xsi??? Which ones are recent and which one is the best in termsof quality?
A: Canon and Nikon's Digital SLR lines:
If you want the best, the professional lines obviously give you the best image quality, build and performance. However,you don't seem to be an ideal user for these camera bodies so I suggest you do what most beginner photographers do, pick up an entry-level body. Youcan't go wrong with a Canon XT, XTi or XSi.
Q: What does DSLR stand for?
A: Google it.
Q: What are the 5D and 5D Mark II??
A: The mark of a Canon camera or lens indicates it's generation. Just like how you'd describe iPod's (1stgeneration, 2nd generation...and so on).
Q: D40, D60, D90 all pertain to Nikons right? But what are they exactly?
A: Yes. Google it.
Q: What is a fish eye? Is it a lense? If I go to Best Buy will the employees call it a fish eye or is that just a nickname?
A: Google it. Yes. I'm pretty sure Best Buy won't have any fish eyes available. They are specialty lenses soyou'd probably have to find them at local or online camera stores.
Q: What does all the "55-80 mm" stuff that describes different lenses on websites mean?
A: It's a lenses focal length. Google it
Q: I know for a Canon, the camera is the body and you have to buy lenses for the body, but can you shoot with just a plainbody or do you have to have lenses?
A: The camera body does not only pertain to Canon. If you Googled what a DSLR is you'd know the answer to thesequestions. No, you cannot take photo with a DSLR using just a body.
Q: Can you put Nikon lenses on a Canon's body?
A: No. However, some lenses can be mounted to the opposite brand's body if an adapter is used. There are 3rd partylenses available for both Canon and Nikon (note: there are other dslr manufacturers out there but these 2 are the giants) at lower prices. Some are good butmost are inferior to the Canon or Nikon counterpart.
If you're gonna ask questions about the definitions of aperture, shutterspeed, iso and other camera settings then forget it. Explanations on these termscan be easily found online. NT is a sneaker forum not a photography forum...so this thread is all we have. As a result we cannot have a stickied thread on theintroduction to digital photography. Most of them can be found by searching on Google and so I suggest you this. We're more than happy to answer, givetips, critique photos people have requested for but if you keep asking simple questions that can easily be answered by searching online then most of us areprobably gonna ignore you. Good luck and have fun with photography if you decide to really get into it.