Job types
The kinds of work you do (Wedding, Portrait, Commercial), tagged on each enquiry.
A job type is the kind of work a job is, a Wedding, a Portrait session, a Commercial shoot. You set up your own list in Settings → Job types, then tag each enquiry with one so you can tell your work apart at a glance (it shows in the Client Tracker).
Job types vs event types
These are different things. A job type is the kind of work. An event type(under Event types & availability) is a meeting slot a client books with you, like a Discovery call. One describes the job; the other is a calendar slot.
Adding, renaming, archiving
Add a job type by typing its name and pressing Add. Rename one any time. Archiving hides a job type from the picker without deleting it, enquiries already tagged with it keep the label, and you can restore it whenever.