Theory of evolution
|We're all Homo here|
|A Gradual Science|
|Plain Monkey Business|
Wanna know more about the theory of evolution? It's funny that you bring that up, because actually, there is no one theory of evolution. There are actually a few, one of which is certainly beyond dispute (namely, evolution by natural selection acting upon random mutation), and some of which are almost completely discredited (Lamarckism, for example, but look up epigenetics for a modern take on the subject). The exact details of how evolution works can be quite controversial (see for example punctuated equilibrium), but most of the time, "theory of evolution" is simply shorthand for "Theory of Evolution by Natural Selection", which is the prevailing theory of how the diversity of life forms came into existence. As such, it's a really, really long story, but it boils down to this:
Almost always, when evolution is referred to as "the theory of evolution", it is being used by creationists or intelligent design advocates and is meant to imply "only a theory". The use of "theory of evolution" over, say, "evolution" or "natural selection" or "punctuated equilibrium" or numerous other actual theories emphasises the "theory". This is done with very few actual theories in common usage (perhaps "string theory" is the most notable exception); for example, "theory of relativity" is usually not used in favor of either "special relativity", "general relativity" (which are the two major parts), or just generally as "relativity". Usually "theory of…" isn't widely used in science because there usually is more than one theory, as described above. "Evolutionary theories" is certainly the most accurate phrase to use.
A bit more about the "theory"
Evolution is real, and observed — we can prove that, moths, finches, dinosaurs, domestic pigeons, horses, whales, Tiktaalik, and all. The question is what makes it work, and we actually have a pretty decent handle on that. It all starts with mutation: the genetic code is chemical in nature, and can be disrupted by any number of things, most of which damage or rearrange the nucleotides in DNA and/or RNA. If the code itself isn't damaged, it can still be altered in duplication (during mitosis, for example), perhaps copying the same section multiple times, or incorporating foreign DNA (such as that from retroviruses). Many of these changes have no net effect on the organism (for example changes in junk DNA, which simply are not expressed), or have no effect on the organism's fitness. But sometimes they do. This is where natural selection comes in.
Whether a mutation is beneficial to the organism or not depends a lot on the environment. Lethal mutations will generally kill the organism during early development (though some genetic diseases strike only after the organism has reached adulthood and passed on the gene in question), but other mutations either increase in frequency, if they improve fitness, or decrease in frequency (sometimes to the point of being "evolved out") if they harm fitness. For example, a gene allele which results in a reduced resistance to a disease will decrease in frequency in a population exposed to that disease (that last part is important and oft-ignored).
Such changes accumulate with each generation, especially as the species moves into different environments or habitats. The eventual result is often macroevolution, also known as speciation (note that, although macroevolution is indeed a scientific concept, its use by creationists is often twisted). And that, inasmuch as a generalization can give a decent snapshot of a complex story, is the Theory of Evolution [by Natural Selection].