"Like" is a casual action taken against any number of objects to commonly express positive or favorable sentiment. It is similar to a positive vote or "upvote", but is defined separately to match common usage habits.
Liking commonly applies to articles, videos, photos, or other kinds of social media.
http://activitystrea.ms/schema/1.0/like
Un-like:
Like is similar to Mark as Favorite, but is more casually and gestural, whereas favoriting tends to have a "bookmarking" connotation.
For related thoughts, see: Learnings about web ratings systems by Adina Levin.
This verb is currently being investigated, and is not part of the core verb schema.