Classes are classification of trees, relevant to group mechanic
Trees are sets of magic within a class that a character can become attuned to
Branches are sets of powers within a tree
Power Tiers are individual spells which, when unlocked, can be cast using a character's energy pool

Like casting a spell, survivors can expend energy to cause various effects. Every creature has powers which come naturally to them, known as a Natural Branch within their class. They master one power in this branch every time they gain 5 flux, up to the maximum of 8 powers at 40 flux. 

Additional branches and the powers within them can also be mastered by aligning Karma at a Ley Line.  On your tracker, you spend this karma in exchange for the power.  In lore, the karma crystalizes within your character, becoming a permanent fixture in your soul.  Each fixture is associated with a Tree, and allows you to channel powers for that tree.  Karma cannot be refunded.

Each character is aligned to one Primary Class (Source, Astral, or Planar), and a maximum of 2 Special Classes, and unlimited additional Condition-Based Classes. Primary Class is an inherent part of their soul, determined at birth, and cannot be changed.

Special Trees are hidden throughout the world, accessible through various origins, in-roleplay opportunities, or developing a unique custom tree on your own.  Special classes will have unique requirements that must be met to become accessible.  They can be abandoned to free up a special power slot.  Special classes have fewer trees and branches than primary power trees as they focus on a specialized purpose.

Condition Based Trees are gained or lost when the associated Condition is developed or cured. Bane based trees offer powerful magic, but reliance on them may worsen your Condition or otherwise cause harm.

Classes: Primary, Special, Condition
Trees: Astral, Planar, Source, Construction, Demolition, Gilded, Warped, etc
Branches: Strength, Telepathy, Telekinesis, etc
Tiers: Smash, Astral Speak, Dexterous Summon, etc

Ex. My Primary Class is the Astral tree.  I unlocked tier 3 of the Telepathy branch, so I can cast Fortify Psyche.

Ex. They have Tier 4 Hydrokinesis, so they can use Water Affinity.  

Ex. His Special Class is the Construction tree because he's from Fort Pipero. He only has tier 1 so far.

This karma cost of powers is a known thing to the characters.  Power purchasing is often ritualistic, and can vary depending on the characters background and faction. Coyotes and cats tend to meditate or hold silent vigils near a ley line. Warren rabbits prefer a social rituals with dancing, storytelling, and singing.  City rats may view their offering as simple procedure, using energy manipulation to force ones karma to align with their magic.  

The experience of gaining a new power can vary from character to character.  Characters particularly in tune with their magic (high Energy Manipulation skill) may feel a rush of heat as they unlock a new fire-themed power.  Characters who struggle with magic may not even know what power they purchased!  This experience is at the roleplayer's discretion; a character who struggles with their powers in character will not see any mechanical repercussions.

A character can unlock as many Branches as they can afford within a Tree they have access to, and may freely purchase tiers 1-8 in branches they have access to.

Tier 9 and Tier 10 powers are limited.  One character can unlock up to 3 tier 9 powers and 1 tier 10 power.  This is designed to encourage specialization, and give characters a true sense of expertise in their favorite branches.  Special Trees may have additional unlock requirements.  Same for Afflicted Trees.  See the power tree description below to check before buy.

Karma may be spent to unlock powers in a character's natural class before they would naturally be unlocked if desired. Karma spent this way will not be refunded, nor accelerate the speed of natural learning.
(e.g, a 10 flux cat will have tiers 1 and 2 in their natural tree, and can purchase tier 3 for 70 karma. At 15 flux, no natural powers are unlocked, as Tier 4 still does not naturally unlock until 20 flux.)

Unlocking powers at unfamiliar Ley Lines comes at a premium.  The magic of the Ley Line is different from what the character is used to, so more time and karma must be spent to access new powers.  This is a persistent problem for Factionless characters, who cannot remain in one location long enough to properly attune themselves to a new Ley Line.