Skill Point Spending Strategies
Take a look at some common Skill Point Spending Strategies

Starting Skills
All 7 player character classes start with the same 2 basic skills - Attack and Throw.

"Attack" attacks with the equipped weapon (or weapons, in the case of an Assassin wielding two claw class weapons, or a Barbarian wielding two one-handed weapons or swords).
"Throw" throws a throwing weapon.

Characters also start with the "Unsummon" skill to destroy minions previously summoned by the player.

Necromancers start with a Wand with +1 to Raise Skeleton.

Number of Skills
Each character class may develop 30 more unique skills. These 30 additional skills are divided into 3 skill tabs (10 skills per tab). The skills in each tab are arranged into a Skill Tree -- much like a Tech-Tree in a real-time strategy game.

Passive and Active Skills
There are two general classes of skills - Passive and Active. Active skills require you to explicitly cast or use them. Passive skills are not cast, but have implicit or automatic effects. Masteries (a type of Passive skill) are in effect whenever the item or skill to which the Mastery applies is used. The Paladin's Auras (another type of Passive skill) are in effect whenever readied as the right mouse skill button. Aura skill button icons are golden.

All Active skills except Attack, Throw, Unsummon and Sacrifice use Mana. Most Passive skills do not use Mana. The Mana cost and other information about a skill is shown, when you move the cursor over the skill icon. Sometimes when you place enough points in a skill it no longer requires Mana or a very low amount of Mana.

Red Skill Icons
When a Skill icon is red; the Skill cannot be used - either because of a lack of Mana, because an item that the skill requires is not equipped, because there is a Casting Delay, or because that skill cannot be used in town.

Synergies
Synergy bonuses are designed to boost the effectiveness of the higher-level skills based upon the number of points allocated to the lower-level (synergizing) skills. Players are rewarded for using skill points earlier rather than hoarding them all for later 'cookie-cutter' distribution to high-level skills. A skill that has a Synergy lists other skills that will help improve the skill you are looking at. This guide lists the bonuses as they are listed in the game so there won't always be complete information about what each Synergy does.

Items that give bonuses to skills and skill levels will not add to Synergy bonuses. For example, an Orb with +3 to Fire Ball, when equipped, does not give any bonus to Fire Bolt. To synergize Fire Bolt, you must place skill points into Fire Ball (or Meteor).

Additional Skill Points
You receive one extra skill point to distribute each time your character reaches another level of experience.

Some quests give extra skill points as a reward. These can only be used one time per difficulty level.

Skill Points from Items
Scepters, Staves, Wands and some class-specific items may spawn with +1, +2 or +3 to one, two or three specific skills for a certain class (see the Items pages). Even if you haven't spent points in them, you will be granted these skills, but only when the item is currently equipped. Skills granted by an item do not allow you to advance further down the skill tree. That is, if a skill has any prerequisites (as indicated by the large shaded arrows in the skill tree) and is not granted by an item, then it will not be unlocked until you place at least one skill point in each of the prerequisites and meet the required level. So an item that gives +1 to Teeth for the Necromancer does not enable you to place one point into Corpse Explosion at level 6 until you put at least one point into Teeth.

Some Items give +X (1, 2 or 3) to a certain class' Skill Levels. This bonus is only for that class and applies only to skills of that class with at least one point (spent or granted by an item). You cannot use these items to unlock all of your skills.

Some Unique and Set Items give +X (1 or 2, usually) to All Skills. This bonus applies only to skills with at least one point (spent or granted by an item).

Some Items give +X (1, 2 or 3) to a certain Skill Tab (Class Only) such as +1 to Combat Skills (Paladin Only) or +3 to Curses (Necromancer Only). This bonus is only for that class and applies only to skills from that specific Skill Tab with at least one point (spent or granted by an item).

Skill Points that work for Any Class ("oskill" Skill Points)
The Trang-Oul's Avatar Set gives +18 to Fire Ball and +13 to Fire Wall. The boost to Fire Ball (as well as Frostwind's Arctic Blast bonus and a whole lot of other items) are examples of what's internally known as an "oskill". These are skill bonuses that work for any class. If a Necromancer is wearing a Trang-Oul set, he'll get +18 fireball and any +X To All Skills equipment can further increase this. The same applies to all other non-sorceress class wearing a part of that set.

However, oskills work differently for classes that "own" that skill. In this case, it's a Sorceress getting a bonus to Fire Ball. In the other case, it was a Druid getting a bonus to Arctic Blast. If a character gets an oskill bonus to a skill that class "owns," the bonus is restricted to a maximum of +3. If a Barbarian is dual-wielding Call to Arms runeword weapons, then the effective maximum bonus from two CtA is +5 (+3 oskill bonus and +1 to All Skills from each Call to Arms). This is all intentional, in order to balance these items.

Skill Caps
You cannot place more than 20 points in a specific skill. You can however rely on skill bonuses from items, Battle Command (temporary bonus: +1 to All Skills) and a Skill Shrine (temporary bonus: +2 to All Skills) to increase the skill level beyond 20.

You can only put another skill point into a skill for each character level past the required level for that skill. For example if a skill requires your character to be level 6 you have to wait until your character is level 7 before you can assign another skill point into that skill. You can however save up skill points until your character reaches the level required for the next skill tier (6, 12, 18, 24, 30) and thus learn several skills at once exactly as they are unlocked.

Skill Respecialization System
In 2010, there was a new talent respec system added that refunds your talent and stat points. There are two ways to respec:

a) Den of Evil quest reward that can be used at any time (max of 3 since you can only complete a quest once for each difficulty)
b) Create a Token of Absolution by combining four different "Essences," in the Horadric Cube which are random drops from the Act Bosses in Hell difficulty:

Twisted Essence of Suffering (Andariel or /Duriel)
Charged Essense of Hatred (Mephisto)
Burning Essence of Terror (Diablo)
Festering Essence of Destruction (Baal)

Right-clicking the token resets all skill and attribute points in the same manner as the reward from the Den of Evil quest. The components have a pretty low drop rate.