Every so often I see a question on SO which forces me to spend half an hour editing a really bad Wikipedia article. The article is now only moderately bad. In the part that bears on your question, I wrote as follows:
In computer science, a function or expression is said to have a side effect if, in addition to producing a value, it also modifies some state or has an observable interaction with calling functions or the outside world. For example, a function might modify a global or a static variable, modify one of its arguments, raise an exception, write data to a display or file, read data, call other side-effecting functions, or launch missiles. In the presence of side effects, a program's behavior depends on past history; that is, the order of evaluation matters. Because understanding an effectful program requires thinking about all possible histories, side effects often make a program harder to understand.
Side effects are essential to enable a program to interact with the outside world (people, filesystems, other computers on networks). But the degree to which side effects are used depends on the programming paradigm. Imperative programming is known for uncontrolled, promiscuous use of side effects. In functional programming, side effects are rarely used. Functional languages such as Standard ML and Scheme do not restrict side effects, but it is customary for programmers to avoid them. The functional language Haskell restricts side effects with a static type system; only a function that produces a result of IO type can have side effects.
