Skins are cosmetic changes for most abilities. Skins may alter aspects of your ability. Here's a list of what could be changed:
The model and the textures used for the model
Effects (particles and other effects e.g. sword slashes)
Idle (the idle pose) for the stand and/or the user
Pose music or/and the pose itself
Animations
Voicelines
Quotes
Skins do not alter the actual moveset nor any features of it (such as cooldowns). You can buy skins from the in-menu shop by purchasing Skin Crates with the use of Universal Shards (u̶S), which you can obtain 400u̶S every day by doing the Daily Quest given by the Daily Quest (NPC) or by simply selling items to the Black Marketeer.
Here is what happens if you equip a skin for the ability that you're currently using:
If you're not in combat you will respawn on the spot (same health and cooldowns).
If you want to equip a skin for your stored (not currently in use) ability:
Nothing will happen, but when you change to that ability it will be changed to the equipped skin.
Crate Pity
Crate Pity is a mechanic that was added in Version 1.8. How it works is that every 100th pull is now guaranteed to be a Mythic skin .
Unusuals
Unusuals is a mechanic similar to the one present within Team Fortress 2, in which there is an exceedingly rare chance when opening skin crates that you might come upon a skin truly unique, the skin differs with 5 possible patterns and 72 possible colors. Additionally their VFX changes color correspondingly on their unusual color.
The World Over Heaven: Party Starter/ OLD UNIVERSE
The World Over Heaven: Galizur/ OLD UNIVERSE
King Crimson: KCR/ OLD UNIVERSE
Asgore: True Asgore/ Asgore Boss (2023)
Umbra: Ama No Murakomo/ ORIGINAL UMBRA OWNERS
Umbra: Mayhem
Dawn: Godbreaker
Dawn: Dusk
Shadow DIO: Shadow Kanna
The World Over Heaven: Fabled Platinum
Weather Report: Bon Report
Crazy Diamond: Bocchi
King Crimson: Carnage
Star Platinum: Shinigami
The World: Kathastrophe/Premium
C-Moon: Scentless Apprentice
King Crimson: END
The World: Worst Low
Star Platinum: Arcade Over Heaven
The World Over Heaven: Arcade
Silver Chariot: Pirate
Silver Chariot: Uriel
Kars: Aureation Blades
Locke: Murimasu
Locke: Clever Curse
PLANET // SHAPER: Sixer PS
Ope Ope No Mi: Average AUT Player
Dragon Knight: Black Knight
Trivia
Originally, the Old Universe didn’t have a skin system, so skins were obtained by using an item on the ability, which was tedious to find.
Though the Unusual feature commonly changes the appearance and VFX color of the skin it applies to, there are certain exceptions in which the skin won't be properly altered by this, making the skin mostly unchanged if not the exact same as if it didn't have the Unusual status. The Unusual's charm, thankfully, will be shown regardless without alteration, making it so people can tell it's authenticity.