Lawns can be anything from a bunch of unrelated weeds that get a cut every now and then to a beautifully maintained area of a single type of grass. For the majority of people their lawn sits somewhere between these two extremes.
Ideally for a lawn to keep looking fresh and vibrant it needs to be mown regularly, the more varied your lawn is the more often it needs a haircut to maintain a pleasing to the eye aspect.
Some lawns, based on vigorous grass types like Kikuyu, buffalo or Couch need to be kept in check, other wise they will turn out to be your biggest nightmare. These grasses are based on a mat of interwoven tentacles and given the right conditions (i.e,. warmth) will not only grow on the lawn but they will invade your garden beds as well and if this happens it can be a major job to get it back under control.
So in summary the easiest way to maintain a lawn irrespective of its makeup is to mow it on a regular basis. The other thing to keep in mind, particularly if you have a strong growing grass like Couch is to make a well defined garden edge so that you can at least see when the Couch is starting to encroach on the garden and quickly nip in the bud.