Someone recently wrote that he didn't believe in evolution because it relies on blind accident. The accident part is true, but it's not the whole story.
Evolution by natural selection refers to the survival (and replication) of organisms that are better adapted to their environment. It's a process driven by mutations -- changes in the hereditary instructions in DNA that occur spontaneously or are caused by radioactivity or cosmic rays.
A mutation that confers a beneficial adaptation is a rare "accident." But consider that a successful adaptation is the product of natural selection working over vast periods of time on vast numbers of organisms, with a multitude of misses and dead ends. Now and then, against all odds, there's going to be a "hit," akin to a particular person drawing the winning lottery ticket. Once that happens, the "accident" part of the story is done. From that point on, the lottery winner genetically passes the adaptation along to offspring in a straightforward, non-accidental manner, with each succeeding generation doing the same. Those with the successful adaptation survive, while those without it may not.
Through this natural process, over millions of years, organisms move from simpler, lower forms to more complex, higher forms.
Don't be misled by the mantra "evolution is only a theory." Think of the process of evolution and the scientific theory of evolution as two separate things.
That evolution (the process) occurs is incontrovertible fact demonstrated by the fossil record and molecular biology.
That's evolution in a nutshell.