Affinity

(Affinity was temporarily removed for a rework in 1.21!)
Affinity is a group of four elements that you can level up and master in order to give certain buff and advantages. 
A more detailed description can be found here.
Your current affinity levels can be looked at with/affinity, /skill, or /mastery.
Affinity comes in these four elements:
Air - Improve your skill in this element by traveling (Either by swimming, walking, or running but not flying). Exp from traveling is collected and added to your levels every ? seconds.
Increasing your mastery of this element will grant you these things (scaled by the level you are at):
Reduces anvil costs
Increases experience orb value
Fire - Improve your skill in this element by fighting creatures and enemies with a close range weapon (i.e. sword, axe, etc..)
Increasing your mastery of this element will grant you these things (scaled by the level you are at):
Increases attack damage
Water - Improve your skill in this element by fighting creatures and enemies with a close range weapon (i.e. bow, crossbow, etc..)
Increasing your mastery of this element will grant you these things (scaled by the level you are at):
Increases health regeneration
Earth - Improve your skill in this element by mining naturally occurring blocks. Certain blocks (such as obsidian) will give more exp per block than others. Blocks placed by a player will not count towards this skill. 
Increasing your mastery of this element will grant you these things (scaled by the level you are at):
Increases damage resistance