This is Path of Exile's passive skill tree. It is a big web of 1350 skills that provide passive bonuses to your character. Each time you level up or complete certain quests, you can allocate a skill and explore deeper into the tree. All character classes share the same tree, but start at different locations that are aligned with their primary specialties. You're able to either focus on improving your core abilities or travel across the tree to allocate exotic skills that your class normally wouldn't have access to.
Alongside the regular passive skills are Notable and Keystone passives. Notables have slightly larger icons, specific names and larger effects. They help guide players to build their characters on a macro level, allowing them to see at a glance what a cluster of passives skills does by examining its Notable passive. Keystone passives, however, fundamentally change the way a character is played by altering the game rules. They usually have one positive effect and one negative effect. For example, Necromantic Aegis takes away the properties of your shield, granting them to your minions instead of to you.
If you want to adjust your passive build during gameplay, there are Passive Skill Refund points available from quests or relatively rare items in the game that can be found or traded for. Fixing mistakes with a character build or improving small-medium sized aspects is relatively easy, but our intention is that players who want to try substantially divergent character builds are encouraged to play a new character through the game, organically leveling it up rather than just respeccing into it.
Heres a link to the Passive Skill tree that you can mess around with (Passive Skill Tree)