Used to be good but was ruined by changes.
You start off on tutorial island where you create your character and learn the basics of Runescape. Tutorial island is relatively boring but is fairly quick so you'll be able to get to the real game soon enough.
Now for the real game. Once you are done with tutorial island you start off at Lumbridge and you are free to do many things. You can choose to do quests which reward you with money, quest points, and/or experience in certain skills. Another thing you can do is train skills. There are many skills you can train including combat or other skills that when trained will help you in some ways. Most of the aspects in the game are controlled by pointing and clicking which allows for easy navigation. Another thing you can try to do is earn money. Their are many ways to earn money in the world of Runescape whether it by using skills or killing monsters. You can also attain a variety of armor, weapons, etc. by spening your "GP".
A year ago Runescape was pretty fun but a few updates have detracted from the quality of the game. First is the wilderness. In the wilderness you were allowed to kill other players at the risk of losing your items. Jagex, the creators of Runescape changed it for reasons unknown. Another thing is the trade. You used to be able to trade any amount but now you can't. You can't give money away or try to get a good deal.
All in all Runescape is a fairly entertaining game that is worth a try.