Both of those terms actually come from a small, but unfortunately very vocal section of right-wing Christian theology that arose in the 19th century. To give a proper explanation of what they do, and do not, have in common with the historical Christian understanding, I need to talk a little mor about Revelation:
Like I said above, Revelation is written in an very, very, very symbolic genre of literature. This genre is called "apocalyptic," which does not mean "sudden catastrophic death from on high" but simply means "revelatory." This genre of literature uses lots of symbolic and poetic language to communicate its meaning, instead of the more usual fact-by-fact historical style that we most naturally employ in modern Western culture. The descriptions that Revelation gives of things to come are to be understood in this fashion if they are to be understood at all. (I could go on about how to go about that, but brevity is the key here, so I won't...)
However, apocalyptic literature is not a genre we use in our culture today, and as a result many people open the book of Revelation and don't know what is going on inside it. They don't understand how this kind of literature works. So, some people treat it like some sort of "history written in advance" that for odd reasons they don't understand uses lots of strange imagery for things. They start trying to "decode" Revelation and look for one-to-one correspondence between the things in the book and literal historical events and people, which is very silly behaviour if one does understand the sort of writing Revelation is.
The terms rapture and tribulation, as used above, come from a particular school of thought that, not understanding what the genre of Revelation is about, treats it as a literal document instead of a symbolic one.
Now, the term "tribulation" does come from the Bible, but the way those guys are using it is an aberration. By ignoring the way the symbols work, they come to the conclusion that there will be a period of exactly seven years when all the stuff described in Revelation is going to happen, more or less as literally as it possibly can. This is, of course, not the right way to understand Revelation, but I've been banging on about that already. In reality, the "tribulation" is not intended to be understood as a specific seven-year period, but the wording in Revelation refers to the tribulations (=trials, difficulties, persecutions) the Christians have undergone and will undergo before Christ's return. So you can see how the way these guys us "tribulation" is aberrant.
The term "rapture" has absolutely no Biblical precedent whatsoever. Remember how I said there would be a resurrection when Jesus came? Well, this school of thought, as a result of poor logic based on their faulty way of reading Revelation and other passages, thinks that this is somehow a two-phase thing. Jesus is going to "sort of" come back once when all the currently living Christians get taken away from the earth, and then later he comes back "to finish the job." This understanding is not only wrong, nor even just wrong from the get-go, but can only arise once one has already been going along the wrong track for a long time already.
So, it is unfortunate that I have to introduce you to eschatology by telling you about things that are wrong, J.F.K., but I guess that is the way the cookie crumbles. If you want to attain a clear and proper understanding, stick with with the simple description I gave you and basically just ignore the likes of Tim LaHaye.
