In Diablo II, each character has 30 unique skills organized into three skill trees. For example, the Paladin has these skill trees: Combat Skills, Defensive Auras, and Offensive Auras. To earn skills, your characters have to reach higher character levels and earn skill points to distribute to learn new skills. Three basic skills are automatically given to your character when you start the game. These are Attack, Kick and Throw. Attack is the basic ability to attack a monster with an item. Kick is not to be used exclusively as an alternative to weapons, because you most likely always will use weapons if you want to actually do a good amount of damage (besides using spells). It is used simply to knock monsters back, and the chances that it will work are about 90%-100%, forcing them back around ten to twenty feet. This actually does do some damage, but it is very minimal. Throw allows you to target a monster and throw an item at it, as long as the other hand (that isn't holding the projectile) is free. However, only certain, "throwing," items may be used with this skill, such as throwing knives, throwing axes, javelins (only the amazon), and some potions.
There are three different types of skills in Diablo II. These are Active, Passive, and Masteries. Active type skills are roughly skills that need to be readied and require mana to use. The Sorceress' Firewall skill is a perfect example of this skill type. Passive skills are almost the opposite of Active skills. Passive Skills do not cost mana, and do not need to be readied in the active skill buttons to have effect. While skill Masteries are Passive Skills that increase proficiency in a given field. For instance, the Barbarian's Weapon Masteries increase his damage and attack rating with certain weapons. It is helpful to know which skills are what types, so be sure to note the information within each of the five character classes respective skill trees." - blizzard.com