The other writeups in this node are a little out of date; they refer to what is now Runescape Classic or RSC, the old version of Runescape that was replaced by what is commonly known by the community as Runescape 2 in March of 2004. RS2 brought with it a complete overhaul of the entire game, both in terms of graphics and gameplay. While the graphics still can't compete with any other commercial MMORPG offering, they are entirely sufficient for the game and no longer induce peals of laughter. The gameplay in RS2 has also improved significantly. Combat has been enriched with the addition of dozens of new weapons and equipment and magic and ranging have become useful combat skills.
Runescape can be played for free, but free players have access to only the free worlds, which are about a fifth as large as members' worlds. Free players also have many other restrictions such as limited bank space, inability to use the most powerful weapons and being unable to use or train several skills. For $5 USD a month, a membership for one character can be purchased. Members can still use free worlds, however most members choose not to unless they are trading with a free player because free worlds are known for an overabundance of beggars, spammers and rather unpleasant people. The Runescape community is, in fact, almost certainly the worst of any MMORPG and may be a contender for the worst of any online game. Generally players that are a lower level than you will beg you for money or items and players that are a higher level than you will ignore you. Any high level player that wanders into a heavily populated low-level area can expect to receive masses of unsolicited trade requests and demands for free valuables. This problem is not nearly as pronounced in member worlds, although you should still expect beggars while in banks or cities used mostly by low-levels. New players, on the other hand, can expect to receive no help whatsoever from veteran players. If you ask politely you might receive directions to the nearest bank or city but tangible assistance in the form of money or items is usually unheard of.
The Runescape economy features a vast gulf between the "haves" and "have-nots". Wealthier players can afford "rares", items that are, for one reason or another, no longer obtainable in-game. Most of the rares are holiday drops, items that appeared at random times on certain holidays years ago. One of the most expensive rares, party hats, were dropped in "christmas crackers" around Christmastime in 2000. Santa Hats, another rare item, were dropped around the same time in 2001. The prices of rares is huge and is always rising- at the time of writing the price of a blue party hat was around 30 million gold. To put this in perspective, one of the most popular ways of making money, fishing for lobsters, will net you about 150 gold per lobster. Players of an average fishing level can obtain about 30 lobsters every 5-6 minutes. While there are much faster ways of making much more money for high-level players, new players have little to no chance of ever being able to afford a rare item unless they are willing to put large amounts of time into the game. However, it should be pointed out that none of the rares offer any gameplay benefit- they merely look nice on your character.
Training most skills in Runescape is a long, tedious process. Training your character's combat skills involves clicking on one monster over and over again for hours or days, or simply letting your character stand there if you are in an area where the monsters are aggressive. You can usually expect to make very little profit training your combat as the best monsters to train on are low-level ones that deal little to no damage to you- the items these monsters drop are usually worth nothing. While there are exceptions they are few and limited mostly to members' worlds.
Training non-combat skills is also a long, tedious process you can expect to make very little profit from. Nearly all skills will make little money until you are very advanced in it; some skills such as smithing simply are never able to make you money at all. Training your non-combat skills usually involves clicking on one thing for hours or even days, such as a rock for mining or a tree for woodcutting.
Nearly all non-combat skills will not help you make serious amounts of money- smithing, for instance, is nearly completely obsolete, as the highest level armor a player can make with it (runite armor) is about the third best, and it would take months of training and tens of thousands of ore to arrive at the proficiency needed to make it. Players who are serious about their moneymaking perform what is known to the Runescape community as "merchanting", an activity wherein one buys as much of a certain resource as possible from other players and then resells it for more. For instance, a merchanter might buy small amounts of coal from various players for 140 each and then, once he has amassed 3,000 coals or so, sells it in bulk for 180 each. (An interesting peculiarity of Runescape that prices go up when selling in bulk.) Wealthier players often do the same thing with rare items, for example, they might buy a party hat for 22.4 million gold and resell it for 22.5. This is entirely possible because there are no absolutely agreed-upon prices for anything within the community, merely an acceptable range.
Runescape has a number of optional quests for players to take; usually they involve delivering certain items to certain people or defeating a certain monster. They can be extremely tedious and a common complaint is that the reward is generally underwhelming. Often the reward is a small amount of experience in a certain skill- many players have complained that the amount of experience they receive is a tiny fraction of what they would have received if they had just spent the time training that skill. There are many more quests available to members than free players. Many of the members' quests open new areas; whenever Jagex adds a new area to the game they will usually add a quest to go with it and require that the quest be completed before the new area is opened to the player.
Despite its myriad problems, Runescape is still very popular: it boasts over 400,000 members, let alone the free players. At peak times there are usually about 100,000 players on the members' worlds and free worlds combined. Why? Well, for one thing the player base of Runescape is on the average younger than that of World of Warcraft or Ultima Online, possibly because Runescape is a very simple game to play. As the other writeups have pointed out the interface is almost entirely mouse-driven and most tasks involve clicking on an object once. Runescape is also much cheaper than other MMORPGs, with a membership fee of only $5 USD monthly. Runescape also isn't that much worse than, say, World of Warcraft- after all, you're stil just killing monsters over and over again regardless of how you do it, Runescape just sort of distills the experience.
If my glowing appraisal of the game has convinced any of you out there to give it game a shot, feel free to contact me- my name is Artie 900 in-game. I've been playing, for reasons beyond my understanding, for about two years now. You may also find the site http://www.tip.it/runescape helpful, as it is the most popular Runescape fan site, and contains comprehensive maps and descriptions of most skills and quests.