Skills are what truly drive character development in RuneScape. Each action can increase a skill and will therefore give some type of experience. For non-fighting skills, the amount of experience gained is set by the action regardless of your proficiency. As you gain levels, it takes more experience to get the next level, therefore making it worthwhile to at least try to do the most difficult thing you can and therefore gain experience faster. There are a number of good skill calculator sites that will show you how many successful attempts you must make of a given type for each skill level.
Your overall level is determined as a function of your Strength, Attack, and Defense - but those increase incidentally to your Melee skill. A newer trait, Controlled, can also be increased through use of Melee with swords and other precision weapons. You can make your own weapons with use of the Mining and Smithing skills. You can also use the Ranged skill to (you guessed it) make ranged attacks, which will also level traits.
Magic is rune-based and Runecrafting is a separate skill. You mine rune essence then bind these blank runes at an altar. In doing so, you tour the world map of the game (which is thankfully not all that enormous) to find the alters and kill whatever baddies are in the way.
Alongside any of these fighting styles, you can also use prayers. The Prayer skill works to buff you in a variety of ways. You can boost any of your traits, grant yourself immunity from certain effects, and much more. I would highly recommend skilling this one up early and often to gain access to the strongest prayers.
Foods are what keep you going in battle. They heal you on the fly (no need to sit an eat like in World of Warcraft) and have no recharge time. You can click on them and just keep on fighting. Some of the best foods to carry are fish. The Fishing skill is fairly easy to start on. Grab a pole and find a fishing spot. There are little swirly pools in waterways indicating that fish are present. Once you have caught the fish, cook them up to level your Cooking skill. You will, of course, need some heat to cook with so leveling Woodcutting and Firemaking together is a good idea. By keeping a Tinderbox in your backpack, you will be able to prepare foods you catch or find most anywhere you go.
Crafting is a skill unto itself that helps every other skill out there. It is divided up into several types, each of which aids a different skill set. Crafting pottery, for instance, allows you to make pots and pie tins for use with higher level cooked items. Crafted jewelry is used in magic and prayer to enhance the effects. As a free member, you can also craft leather and sew leather armor for ranged-specialized characters. There are other crafted items available for paid accounts as well.
In case you hadn't gotten the hint from the numerous links to the RuneScape Game Guide, there is an extensive knowledge base available for you on the site. The Jagex team has done a phenomenal job of keeping this game fresh for players by adding new skills, new quests, new features, and enhanced graphics. Just make sure you sign in quickly because the new intro background can really make your head spin.
So if you've been looking for a good MMO to try without the pressure to buy or just want something semi-mindless, give it a go. RuneScape is like the Snood of MMOs - really just a clicky game, but oh-so-entertaining!