Skills are a player's abilities

Skills are a player's abilities that may be developed throughout the game. Skills are trained by various actions which use that skill, which give experience in the skill, until enough experience is earned for the next level. Some skills allow players to cook, chop down trees, make fires, use magic, make runes, and so on. Some skills support each other. For example, logs obtained from Woodcutting can be used for Firemaking, and fires from Firemaking can be used for Cooking food, and food can be used to replenish life points to help in Combat. The higher your levels are the more of the skill you will have access to.
There are currently 24 skills in RuneScape. Members have access to all 24, while free-to-play can only advance in 15 of them, with less advantages. There is currently a 25th in production.
You can also train a skill at the same time you gain money! For this, please refer to the Money-making guide, though, is your decision.
A player using the Fishing skill.All skills but Constitution start out at level 1. Constitution starts out at level 10. Players can advance a skill to level 99; after that, they can increase their experience up to 200,000,000 - but gain no more levels for doing so. A player's combat level begins at 3, because of 10 Constitution. The maximum combat level a member can reach is 138. Non-members can reach up to level 126 because a non-member can't train the Summoning skill.See the hiscores for the relative rankings of the different skills.
If a knowledgeable player were to start a new character, with certain assumptions for efficiency in activities like training combat and summoning with slayer, it would take approximately 3600 hours to go from all level 1 skills to all level 99 skills.
Skills can be temporarily boosted or lowered by using certain items or equipment, such as Capes of Accomplishment, beer, potions, and pies.
If a player is in the top 2,000,000 players in a skill, and is of level 30 or greater in that skill, they will appear on the hiscores.
It has also been confirmed by Mod MMG that a new skill will be introduced to RuneScape sometime during 2010. Which skill this is, however, has not been announced. On Monday the 25th of January Mod SteveW responded to a comment on the Runescape Channel saying that the new skill would be released in a month with 28 days. The next day, Tuesday the 26th of January, Mod Crow posted on the YouTube Runescape Channel the following: "Hi all, just thought I would quickly clear up something. It appears no one really got Mod SteveW's attempt at humour when he said it will be in a month with 28 days in it. ALL month's have 28 days in them! It was a cheeky way of saying 'No sneak information' from the Steveanator." This seems to be a confirmation that the skill will be released in February, however, all months have at least 28 days, so Jagex likely did not want to set any sort of date for the release.
Mod Fetzki said in the Q&A that this new skill will take free players beyond the free player map