1.
Linear functions happen anytime you have a constant rate of change and there is a proportion between two things.
A constant rate of change is recognized because has the form 'x' per 'y' or 'x' for every 'y'. Any case that is not linear is, then, nonlinear.
2.
Suppose bananas cost $0.5 per pound or a car travels at 50 miles per hour. Here cost is proportional to pounds and the distance travelled is proportional to the time travelled, and the rate of change is constant in both cases.