Generally it is considered one of three things. A Theory as a Model of the Universe, an outright lie to pry the Country from Gawd's capable hands, or a great euphemism for a steaming one night stand, orgy, or just the best orgasm ever. You Decide.
Essentially, The Big Bang Theory is a work in progress. The whole thing is a changing array of comprehending the Universe as it is known. So it can be a map, a means of understanding the thermodynamic and material nature of the exraterrestrial realms, and it is a way of reasoning and problem solving for whenever something new presents itself. Or it could be the icy breath of being ripped from the book of life and etched into the book of death by a boney fingered 35 year old spinster in the "prime of her life" piecing your eternal damnation like so much scrapbooking scraps. You know, freedom of religion and all that shit. Oh, but the actual body of the theory is that the Universe is constantly expanding and developing. That's right, just like Uncle Cleavus' soft spot in his skull, the Universe isn't done developing either. However, while Uncle Cleavus' skull might not develop at all, if the Universe is done developing, there is a slight chance that everybody will fucking die. Bottoms up, bitches.
The whole theory is best summed up by just about every professional you ask about it as a "Model of the Universe". However, it was originally proposed as a model of the "initial conditions and development" of the "Primeival Atom" by Georges Lamaitre (yes...a french guy). It may have started out that way, but in one fell swoop it became the exact same thing for the whole Universe itself. Not just our Planet, not just our Solar System but the whole Universe. Why? Because it seemed like a goofd idea at the time. Of course, when you and your peers have that kind of fleeting confidence, you don't get a scientific theory proven right, you get a lifetime in prison as a taint scrubber for a big black man named Molly. In reality though, Frenchie didn't really make the math of the model, Alexander Friedman did. Lamaitre just proposed it all. Then, Edwin Hubble discovered that shit was drifting away from other shit because it was glowing with the intesnity that shit glows when it wants to get the fuck away from other shit. Shit! With "Big Bang Theory" proven, everybody knew the score, and that was that the Universe was constantly expanding, much like your Mom's girth for nine months after prom night, and more matter was being produced to fill the gaps.
The reason why the theory changes is because of shit like Dark Matter, which was disucssed in a Cracked Article somewhere, and the arguements that were spat around before the theory was proven. The actual mechanics and the reason behind the expansion is just as much a debate as the phenomenon itself was 90 years ago. So just about anything that is going on in astronomy is going to have something to do with Big Bang Theory just like everything in biology is going to be involved in some way with Evolution Theory. Need an example of this logic?: Think about how your friends have a theory that you use an entire roll of wettened toilet paper to masturbate. Now if Edwin Hubble had caught you on his telescope masturbating with a soaking roll of Charmin Ultra then your friends would know that they were right. After that, whenever too much toilet paper is gone, they are going to know that it has something to do with "Frank Masturbates with Wet Charmin Theory". Oh,a real world example. How about that every thermodynamic breakthrough gives a new perspective (usually simply adding to the old one) about what it is like to be a piece of rock in space, or some shit like that. With each new perspective comes new insight about Big Bang Theory. With Biology, every new concoction is made from knowledge of micro-organisms and the human body which would not be as clear or concise were it not for study in evolution.
For some reason, this lead to assloads of misunderstandings and unnecessary bastardizing summaries to make a cool point on History Channel science shows about the planets and whatnot. Therefore, a whole bunch of religious zealots decided to bitch about it and try to replace it and Evolution Theory with "Intelligent Design Theory", completely ignoring the fact that no matter what subject one hoards this heap of mental abortions with a "Theory" label on it into, it will never actually be a theory because it has no hypothesis, no actual laws to use for testing the hypothesis that never was and no way of predicting what can or cannot happen under the "theories" premise (yes, you have to state both). So I guess you know a good theory when even the lazy asses who want a one-size-fits-all everything (diet, medicine, designer clothing......science) are trying to make a half-ass clone of it, only in a more acceptable package.