What is an Activity?
An activity is a distinct, scheduled piece of work with a duration, resources, and usually dependencies — the unit the schedule is built from. The chain: work packages (from the WBS) get decomposed into activities, activities get sequenced into the network, and the network becomes the schedule.
The boundary worth knowing: work packages are deliverable-oriented (nouns); activities are action-oriented (verbs). "Foundation" is a work package; "excavate," "form," "pour," and "cure" are its activities.
Worked example
The work package "equipment room fit-out" breaks into activities: install cable trays (3 days, 2 electricians), pull power feeds (2 days, after trays), mount and terminate panels (4 days), test and energize (1 day, after everything). Each has an owner, a duration, and a place in the chain — which is exactly what the scheduler needs and the WBS alone can't give.