Evolution is the process whereby a species changes over time through natural selection. Within most species exists genetic variation which is always being added to by mutation, and those variations which enable an individual to survive are then passed on to succeeding generations. Those variations which do not enable an individual to survive are filtered out when the individual fails to survive, and the species lives happily ever after unless something goes wrong and everybody dies. That's it in a nutshell.
Evolution usually slows down dramatically when a species reaches the point of technological advancement that allows previously unfit individuals to survive. Weaknesses that once spelled a death sentence can now be rectified with the modern medicine, decreased need to scrounge for food, and so forth. Of course, in asexually reproducing species like the Jamyns, or species like the Reptons that are now made entirely of clones, evolution has an infinitesimal effect at best. And on planet Veg, the element devolvium brings the process to a virtual standstill altogether.
The term "evolution" is often misunderstood and therefore sometimes misused. For instance, Koozebanian Phoobs speak of themselves "evolving", but all they really do is permanently shape-shift in the presence of danger, which isn't really evolution because it only involves one organism at a time. Similarly, Pokémon are said to "evolve" when they transition through life stages, but similarly, they really don't. A lot of people are stupid that way.