SlowlyGetting startedThe Themer
Template parts
Reusable pieces like a header and footer, shared across templates.
A template part is a piece you build once and reuse across templates, a header, a footer, a sidebar. Place the part in any template, and a change to it updates every template that uses it.
It is the same idea as a fragment, at the level of the big shared regions of a site. Headers and footers are the usual examples.
Like templates, parts belong to the active theme.