What is the difference between Evolution and Natural Selection?

The theory of Evolution by Natural Selection was first proposed by Darwin in 1859. Evolution is the observation that species change over time, for example the beak of birds getting larger or the neck of giraffes getting longer. Evolution can be caused by a number of processes, one of which is natural selection. This is where a non-random subset of the species does better than the population average, producing more offspring and passing on their traits to the next generations. For example, if there is selection for a long neck, then giraffes with longer necks will produce more offspring, and so over generations, longer and longer necks will become prevalent in the population. So, natural selection leads to directional evolution, called 'adaptation', where species seem very well suited to the envrionment they live in. 

However, evolution can be caused by non-directional processes: 1) Genetic drift, where random events lead to the loss of particular traits in the population, for example all blue snails randomly being wiped out in a storm, and 2) Gene flow, where migration of individuals in and out of the population introduces new traits, changing the distribution of that trait value in the population. Therefore, natural selection can cause evolution, but the two terms are not interchangeable, since evolution can occur without natural selection.