Following define an asymmetric relationship between a subscriber and an activity producer.
For the official specification see: http://martin.atkins.me.uk/specs/activitystreams/activityschema#follow
A "followee" (or activity producer) may be a person, but may also be a social object, like a photo, a webpage, or anything else that emits a stream of updates or activities.
http://activitystrea.ms/schema/1.0/follow
Friendfeed uses the "subscribe" verb in place of follow.
Negative activity is unfollow.
There are several well-known implementations, including: