For a list of every Skill obtained from Cards, click here.
Every enemy in the game has a 12.5% (1/8) chance to drop their respective Basic Card. Upgrading a Card to the next tier will always require 5 Cards from the previous tier.
The current tiers of Cards in the game are: Elite and Super.
Obtaining the Elite Card of a respective enemy will allow using their Skill, and can be enhanced further by obtaining the Super Card of that respective enemy.
A Skill, when upgraded, typically increases Damage and SP costs.