Mowing is effectively a system of pruning.  The grasses, which constitute a lawn of any kind all have a natural growth habit.  This may be from a few centimetres to over a metre in height.  To encourage them to adapt to being part of a lawn they require pruning or mowing.  Pruning in its most extreme form can be seen in topiary or bonsai where shrubs and trees are trained to exotic forms or in the latter case maintained in miniature form to a great age.

So it is with grass.  The action of mowing encourages the grass plants to adapt to an alien growth form.  The key elements in this process are height of cut and frequency of mowing.  Height of cut will generally be determined by the purpose the lawn is put to.

• How quickly do I want to cut my lawn?

If your lawn is bumpy and uneven then the best option is a hover or rotary lawnmower as cylinder lawnmower will often damage a bumpy lawn, as they are unable negotiate steep contours.
If your lawn is square with few obstacles then width of lawnmower is not an issue.  However, if your lawn has grass paths, flower beds and trees to navigate, a narrow easy to handle lawnmower will work best.