Golf is played, for the most part, without the supervision of a referee or umpire. The game relies on the integrity of the individual to show consideration for other players, care for the course and to abide by the Rules. All players should conduct themselves in a disciplined manner, demonstrating courtesy and sportsmanship at all times, irrespective of how competitive they may be.
Etiquette is an integral and inextricable part of the game, which has come to define golf's values worldwide.
Put simply, it is a series of guidelines that exist to show other players, whether through divot repair or awareness of your shadow, a degree of fairness which you would expect to receive in return.
In terms of golf's environment, etiquette is about showing respect for the course on which you are playing and the work that has been put in to create it. It's about making sure that the game is played safely and that others on the course are able to enjoy the round as much as you. In short: it's about showing consideration to all others on the course at all times.