Evolution is 'a process in which something passes by degrees to a different stage'* The
biological definition of evolution is 'the sequence of events involved in the evolutionary development of a species or taxonomic group of organisms'*.
Biological evolution was first identified by naturalist Charles Darwin, whose book 'On the origin of species' was published in 1859. It documented Darwin's newly realised belief that the diversification of life on Earth had resulted from a process of gradual changes in species, brought about by natural selection. For example, if you have a bird that needs a long beak to extract food from a flower, then those members of that species who have the longest beaks are more likely to be chosen by mates, leading to offspring who are more likely to share that characteristic of having a long beak. Darwin came to his understanding on evolution following a visit to the Galapagos islands. He noticed that animals, such as birds and tortoises, were slightly different depending on which island you were on. Rather than being separate species, it was as if they had developed differently due to being isolated from each other.
Richard Dawkins in his book, 'The selfish gene', is worth reading for a better understanding of how life first emerged and what elements determine the way that a species evolves. He talks about how the first life would have come about when organic substances (such as amino acids) became locally concentrated and under the influence of energy (e.g. ultraviolet light) combined with larger molecules. Most of the resultant molecules would have been unstable and wouldn't have lasted but, amongst all the unstable molecules, some more stable ones would have emerged and amongst them some would have had the ability to replicate. All life descended from these initial replicators, which over many millenia became increasingly sophisticated.
Evolution is one of those things that makes absolute sense but which took mankind a long time to come to an understanding of. (Indeed, there are some people who continue to dispute its veracity despite the overwhelming evidence). The great thing about evolution is that it shows how connected we are to all other living organisms. Indeed the basic components of DNA are the same whether we're talking about humans, chimps, fish, birds, bananas, etc. It's just the order of them that differs.