The Theory of Evolution began with Charles Darwin and his book, Origin of Species. Interestingly, he did not conclude that evolution began billions of years ago in a chance combination of just the right chemicals. He simply observed that many species were similar enough that they appeared to come from a common ancestor. Despite its "evolution" into what it is today, it has become the default scientific explanation of the origin of life.
To understand evolution, one must understand the difference between the origin of life and the tiny changes that we do see happen and are clearly happening. The origin of life is thought to have taken place billions of years ago when the earth was rather young and was bathed in water and a plethora of simple molecules that included carbon, nitrogen, oxygen, iron, chlorine, etc. In other words, the ingredients of life. It is believed that with the aid of energy from the sun, these chemicals combined in various combinations and every once in a while, or maybe just once, the one in a million chance (so to speak) would cause a combination that could be classified as primitive life giving molecules. Eventually these combined to make more complex molecules until they formed into what would be considered a very simple cell of come sort.
Now of course it is thought that even basic molecules were very uncommon, and the more complex they became the lesser and lesser chance they had of forming, but that the one in a billion billion billion billion, etc. ended up as a cell. Of course this took place over millions of years, it is not supposed to have happened in a short time. However, for some random reason, a tiny fraction, or perhaps even just one of these few primitive cells, through pure chance, happened to have the ability to divide, which it began doing. Soon there were lots of these cells and as they filled the ocean, natural selection began to take effect and most of them died off, but every once in a while, one would have a mutation that allowed it to adapt a little better and therefore reproduce.
It is this natural selection that we see every day. We know for example that wolves were the original ancestors of dogs. This is an example of evolution, just evolution that humans forced. Every once in a while, a wolf was a little tamer than normal and then bred to produce offspring that were also tame. Over many generations, they became what we call dogs today. There are also clear similarities between animals in different places because their environments demanded slightly different survival properties.
These distinctions are important because no one is positing that species can't change over time, but many deny that life could have come from a pool of non-living molecules purely by chance. So when you say that you don't buy into evolution, you don't usually mean that minor attributes of species cannot change over time, you mean that you don't think life originated with a pool of slime. Not understanding the difference can make it seem that you do not know what is evolution.