Statistics, or player stats, augment combat abilities and are divided into two groups: primary and secondary. The primary stats, (Strength, Dexterity, Intellect, Wisdom, and Endurance) are the most vital and each role characters play will require heavy investment into that stat. The secondary stats (Haste, Critical Chance, and Expertise) are still important, but they will be, for the most part, interchangeable and serve as a way to customize and augment your character.
Strength is the measure of one's physical brawn and force, best suited to tougher melee fighters. Strength is used by Reavers, Justicars, and Blackguards.
Dexterity is the measure of one's physical agility and nimbleness, best suited to faster melee fighters and ranged specializations. Dexterity is used by Paragons, Marksmen, and Assassins.
Intellect is the measure of one's arcane power and magical ability, best suited to spellcasters. Intellect is used by Pyromancers, Psions, and Stormbringers.
Wisdom is the measure of one's mental strength and compassion, best suited to healers. Wisdom is used by Cabalists, Invokers, and Oracles.
Endurance is the measure of one's constitution and physical stamina, best suited to tanks. Endurance is used by Champions, Paladins, and Wardens.
Haste decreases the cooldowns of abilities, and decreases the time between attacks (if melee or ranged), OR decreases the time taken to cast spells (if magical). Haste is universally useful.
Critical Chance increases the chance to critically increase the effectiveness of an ability, which doubles the damage or healing. Critical Chance is universally useful.
Expertise increases the chance to block incoming attacks (if using a shield), parry (if using other melee weapons), and dodge (if not using any melee weapons). Expertise is most useful to melee specializations, especially tanks, but still has applications for ranged specializations.