In addition to what BennyC said...
If you want a healthy looking lawn with minimal effort, cut your grass on the highest setting your mower is able to and cut it once a week minimum. My mower has 7 settings, 7 being 70mm height IIRC. Once all grass has reached the same height it looks great and when you stand on it, it feels very nice and soft. I personally leave the clippings on the lawn every 2-3 weeks, providing the lawn has not grown excessively.
It will also help stop seeds getting to the soil so easily and when they do, the grass will create shade over them to restrict their growth. Regular mowing means you cut the weeds heads off before they can pollinate and spread more seeds on your lawn (Dandelions being the worst IMO).
Most people will cut the lawn too short and then it will ruin the lawn and take a long time to recover. I specifically told my brother to do the above with his new turf, yet he cut it short in the middle of summer last year because he 'didn't want to cut it weekly' and it looks terrible
A brand new healthy lawn now looking like a neglected lawn.
