From my understanding, a skill is a package of instructions/resources that Claude applies whenever relevant. They're reusable and provide Claude with domain-specific expertise (i.e. workflows, context, best practices).
This week I translated a ChatGPT Agent I had built into a Claude Skill and it'll be 1 out of the 3 skills that I want to build as part of a larger project we're working on.
Still a newbie, but I also found out that a plugin lets you package skills, agents, hooks, etc. and can be shared across teams and projects