Evolution is the idea that one species of animal can gradually turn into new species. The original idea was that an animal like a giraffe might have evolved to have a long neck from a similar kind of animal with a short neck just by stretching its neck a lot as it tried to nibble the leaves of trees over its head. Every generation of this species would somehow be born with a longer neck as a result of the stretching, and eventually this species of animal quite simply became a giraffe.
indentCharles Darwin thought up a much more likely mechanism to explain how evolution might work, and he called this "survival of the fittest". Some of the short-necked animals would naturally have slightly longer necks than others, and they were better suited to survive in an environment where the best food was high up, so in times when food was scarce, they survived while their relatives with slightly shorter necks starved to death because there were still leaves high in the trees long after the lower ones had all been eaten. The next generation of these animals had longer necks on average than the previous generation for the simple reason that their parents survived the hard times thanks to their longer necks. The new generation of this animal also had a range of slightly different lengths of neck, and the next time food was scarce it was again the longer necked ones that survived, while the rest died. Again the next generation of giraffes had slightly longer necks on average than the previous generation, and so on it went over many generations until their necks were so long that they had become giraffes. The animals which happened to be the most fit for survival in the local environment were always the ones most likely to survive when times were tough, and any trait they had which gave them an advantage over the rest of the herd would be passed on to the next generation while the inferior animals simply died without having any babies.
indentDarwin believed that evolution occurred very slowly over hundreds of thousands of years, but it's actually quite difficult for a successful species to evolve, and they can remain virtually unchanged over tens of millions of years. The dramatic changes needed to create totally new species are most likely the result of rapid environmental change where the species is pushed right up against the edge of extinction: it is only then that the major changes can be passed on to the next generation without them just being diluted out again as it would be with a large population of animals, and it is therefore likely that repeated inbreeding also has to be a part of the process.