The Legacy system starts with three trees: Adventure, Resourcefulness and Professions. Legacy Points are earned across the account, but each character chooses how to spend its own allocation, which lets an alt use a different setup from your main.
Focused on the journey and leveling. Blizzard has highlighted perks such as Well Rested, which improves rested experience benefits.
Utility-focused choices, including deeper perks intended to reduce friction in everyday class and world gameplay.
Profession-oriented perks such as Bountiful Harvest, designed to improve gathering and support profession progression.
Complete Legacy Challenges to earn Legacy Points. The points are shared account-wide, while spending is handled separately on each character. That means earning a point on one character increases the pool available to your account, but your Warrior and Mage can still choose different Legacy Tree allocations.
Blizzard says Legacy Challenges are built around normal play, including leveling classes and reaching profession milestones. Pre-release data also shows goals tied to PvP, reputation, exploration, dungeons and raids. Use the Legacy Challenges tracker for the current reviewed list and category breakdown.
At launch, each character can spend up to 16 Legacy Points in its trees. You can still complete challenges beyond that spending limit and earn up to 65 Legacy Points at the start. The extra earned total also advances the account-wide Legacy Reward Track.
Spending points on a character and earning points on the account are separate concepts. Your total earned points feed the reward progression, while the 16-point launch cap controls how many perks an individual character can activate. See the current cosmetic milestones in the Legacy Rewards tracker.
References: Blizzard Forever Deep Dive · Wowhead Legacy System overview.