Skill

Skills are various class-specific abilities that modify a character's abilities. By choosing a particular combination of skills, a player customizes their character, making that character more effective for certain playstyles.

Skill Points
A player chooses skills by investing skill points, awarded as a character increases in level. Each time that the character increases in level, they receive an additional skill point. The first skill point is gained at level 5, and must be used to acquire the Action Skill for that character's class. Subsequent skill points may be invested in a number of different skills that unlock as the character progresses.

A player may invest multiple skill points in a single skill, increasing the character's effectiveness in that skill. Each skill other than the Action Skill can receive up to 5 points.

By equipping Class Mods, a character will receive additional points in one or multiple skills. For the bonus to any skill to take effect, the character must have at least one point already allocated to that skill. Use of a Class Mod may give a character an effective rank in a skill of greater than 5.

If the player makes a mistake while investing skill points or merely changes their mind, they can spend money at a registration (New-U) station to "respec" (redistribute) a character's skill points. Upon purchasing a respec, all skills are reset, including the Action Skill, and the player must reallocate all that character's skill points. The cost of a respec increases as the character's total number skill points increases, but will never exceed $15000 even for a maximum-level character.

Action Skills
Each character has a single Action Skill. This is the first skill that is learned, acquired at level 5.

Brick
Brick's Action Skill is Berserk. While in this state, Brick's vision becomes clouded with blood, he starts laughing maniacally, and he puts his guns away in favor of using his fists to pummel enemies. His melee damage is increased, he takes less damage from enemy attacks, and he regenerates health in battle.

Lilith
Lilith's Action Skill is Phasewalk. When Lilith uses Phasewalk, she turns invisible and can move quickly. When Lilith goes into Phasewalk, there is a small shockwave that damages enemies in the area of effect. Skill points can be used to increase the melee damage whilst in this state, and to limit damage when coming out of it.

Mordecai
Mordecai's Action Skill allows him to send an alien pet, a falcon-like bird named Bloodwing, out into the fray to attack enemies.

Roland
Roland's Action Skill is Scorpio Turret. This is a turret that attacks enemies while providing cover for the team with an attached shield. It can be upgraded to supply teammates with health and ammo.

Skill Trees
Beyond the Action Skill, access to the different character skills is regulated by a tree structure. Each class has three distinct skill trees. Each of these trees comprises seven different skills, arranged into four successive tiers; the first three tiers of a tree have two skills each, while the last tier only contains a single skill, presumably the most powerful one.

Initially, the character has access only to skills in the first tier of each tree, unlocked by the Action Skill. For every five skill points invested in a particular tree, another tier in that tree unlocks, allowing skill points to be invested in new skills. Skill points may be invested in any combination of the skills of the available tiers: maxing a single skill by investing five skill points in it is possible, but not necessarily required to advance. Likewise, characters need not dedicate themselves to a single skill tree, but may distribute skill points between trees as a player sees fit.

Unlike the Action Skill, the skills in these trees require no explicit command by the player. They either take effect automatically under particular conditions (e.g., when the character scores a kill, as in Mordecai's Killer, or when the character inflicts explosive damage, as in Brick's Liquidate), or else are permanent effects (e.g., the bonus to shield capacity of Lilith's Diva, or the reduction of action skill cooldown time of Roland's Deploy).

Brick
Brick's skill trees are Brawler, Tank, and Blaster.
 * Brawler focuses on melee, especially using Brick's Berserk fists.
 * Tank focuses on damage mitigation and recovery.
 * Blaster focuses on weapons, particularly Explosive weapons.

Lilith
Lilith's skill trees are Assassin, Elemental, and Controller.
 * Controller focuses on mitigating damage and dazing enemies.
 * Elemental focuses on interactions with tech effects and Elemental Damage.
 * Assassin focuses on doing damage, especially with Phasewalk and melee attacks.

Mordecai
Mordecai's Skill trees are Sniper, Rogue, and Gunslinger.
 * Sniper focuses on effectiveness with sniper rifles.
 * Rogue focuses on improving Bloodwing and the loot gained from enemies.
 * Gunslinger focuses on effectiveness with pistols.

Roland
Roland's skill trees are Infantry, Support, and Medic.
 * Infantry focuses on offensive ability with weapons and Roland's turret.
 * Support focuses mainly on defenses and providing extra ammunition.
 * Medic focuses mainly on health and keeping the party alive.

Interactive Skill Trees
Borderlands Official Website offers an interactive skill tree for each character. Note that talent points gained through DLC completions are not used in the talent calculator, meaning a maximum of 57 points can be allotted into the trees.
 * Brick
 * Roland
 * Lilith
 * Mordecai