Skill

"Lidda the rogue can walk quietly up to a door, put her ear to it, and hear the troglodyte priest on the other side casting a spell on his pet crocodile. If Jozan the cleric were to try the same thing, he’d make so much noise that the troglodyte would hear him. Jozan could, however, identify the spell that the evil priest is casting. Actions such as these rely on the skills that characters have (in this case, Move Silently, Listen, and Spellcraft)." -- Player's Handbook

Skills allow a character to measure what they are good -- and poor -- at doing. Each skill has its own primary attribute.

Skills
Strength	Climb Jump Swim
Dexterity	Balance Escape Artist Hide Move Silently Open Lock Ride Sleight of Hand Tumble Use Rope
Constitution	Concentration
Intelligence	Appraise Craft Decipher Script Disable Device Forgery Knowledge Search Spellcraft
Wisdom	Heal Listen Sense Motive Spot Survival
Charisma	Bluff Diplomacy Disguise Gather Information Handle Animal Intimidate Perform Use Magic Device
None	Speak Language