A function is a relation between a set of inputs, called the domain and a set of permissible outputs, called the range, with the property that each input is related to exactly one output. The output of a function $f$ corresponding to an input $x$ is denoted by $f(x)$.
In this wiki, the domain and range are usually subsets of the real or complex numbers.
If $D$ is the domain and $R$ the range of a function $f$ then the standard notation is $f:D \rightarrow R$.