Evolution is the process by which a given system gradually changes. When applied to biology, the term most commonly refers to the Theory of Evolution, the scientific theory which describes the process by which species adapt over time, as observed in the fossil record and laboratory testing. Originally proposed by the British naturalist Charles Darwin in his work On the Origin of Species, it has since been refined and expanded by generations of biologists.
The basic principle of evolution is that a beneficial trait, emerging through random mutation or recombination in a given species' DNA, will propagate itself into following generations, while undesirable traits are not passed on. This is made possible by the fact that these beneficial traits increase the chance that their carrying creature survives to adulthood to procreate and pass on said trait. A common misconception is that this leads to survival of the fittest, ie those creatures who are physically superior; in practice, it leads to the survival of those creatures who are most suited to their environment, regardless of physical prowess.
When populations are isolated through geographical or climate factors, a new species may arise as more unique traits are passed on to following generations. A prominent example - and one that partially inspired the original theory - was the development of subspecies among the finches of the Galapagos Islands, where populations of birds had evolved different beak structures to better gather specific types of food.
Current status and controversy
Within the scientific community, evolution is virtually universally accepted as the means by which the current diversity of species on Earth came into being. There are, however, small groups which still resist the acceptance of the theory, usually on the grounds that it conflicts with their religious account of the creation of Earth. These groups have marginal influence within the scientific community; instead, they often resort to presenting their views directly to laymen, preferably by introducing their own views (see Intelligent Design) directly through the school system. These efforts have had some success, particularly in the United States.
In Science Fiction
Many science fiction franchises, Star Trek in particular, portray an extremely distorted picture of evolution theory. The words "Evolutionary Level" will most likely be used, implying some forms are inherently more advanced.
Aliens and humans often undergo a metamorphosis into some kind of different creature, with the event described as an "evolutionary" change. Real evolution occurs from one generation to the next, as different traits are selected by natural selection in different proportions in subsequent generations. Metamorphosis is not evolution.
If the misunderstanding doesn't involve sudden transformations, it probably subscribes to the notion that humanoids are some sort of goal of evolution, hence all advanced species are humanoid, and everything else is evolving toward a humanoid form. Real evolution does not "advance" toward a particular goal, it simply filters out forms that don't reproduce successfully in their environments.