Changes in the heritable attributes of populations of organisms over time. The mechanisms of evolution are mutation, migration, drift, and selection.
Evolution is a change in gene frequencies, resulting from four mechanisms (mutation, migration, drift, and selection) which can all affect the transmission of heritable genetic information within populations.
Charles Darwin described evolution with the term "Descent with modification" to illustrate that (genetic) information, which can affect phenotypic traits, is inherited from parent to offspring, with the potential to be modified or altered with the passage of time.
Questions on macro- and microevolutionary processes, speciation and divergence, the process of adaptation, effects of population genetics, etc. are generally on-topic. Questions about whether evolution is true, and other questions which would tend to illicit highly opinionated and poorly researched answers, are generally off-topic.
Before asking why a seemingly advantageous trait has not evolved see this post.
Understanding Evolution is a particularly useful introductory source and includes a large myth-busting section.