what are funtions . how can function defined.how can we find inputs and outputs in graphs

Function is a relation which shows the dependency of values of one set to the other set. The set of values which follows the relation is called domain. All the values produced by the function is called range.
The graph is generally defined as .
Here, x is input which is generally expressed at horizontal axis on the graph and f(x) is output which is generally expressed on vertical axis.