Merge selected files or folders

I'd like to merge only certain files, or folders, from another branch.  What command or options should I be looking at to get this done?

What keywords should I be looking at?  When I ask Google I seem to find various answers which then get analysed as "actually don't do it like this, because it doesn't actually do a merge" or other unsatisfactory results.
My colleague is mostly working on "master" and I am mostly working on a branch.  In most cases my branch should fast-forward to the head of master, so I'd have thought that behind the scenes it would just need to re-point the commit identifier, so there ought to be a straightforward implementation of this.

I know git expects to merge a whole branch throughout the repository, but this seems like something I can't be the only person ever to want, so I assume there is a "proper" way of doing it.  Or do I merge all then clear everything I don't want, which seems tedious.

Regards,
Richard.