As blue112 noted, the dividing line is fuzzy. The very first documentation page, however, has an explicit list (and as R.M. notes below, one primary criterion is, or at least is supposed to be, stability of interface—for which some nominally-porcelain commands have --porcelain1 to force a more stable and/or more machine-readable output). You can choose to use their list, or decide that some commands are too high level to be low level, or too low level to be high level. For instance you might disagree that git apply is a plumbing command, yet the Git page says it is. Or, you might consider things like git fast-import to be something you would only use inside a script.
Edit, 31 May 2020: The Git documentation has changed since 2016 to reassign commands to new sections; the data below are now out of date. Consult the documentation for your own Git version (via
git help gitfor instance) to see what your own system says. See also VonC’s answer with a link to a 2018 change to the documentation.
The list below is simply extracted from the documentation, with descriptions and additional classifications stripped to leave only “porcelain” vs “plumbing”. (Sub-classifications remain visible as inversions in the alphabetic sort order. I did not construct links for each entry, as this would be considerably more difficult with StackOverflow markdown—this just needed a simple <pre>…</pre> wrapper.)
porcelain
git-add git-rebase git-cherry git-am git-reset git-count-objects git-archive git-revert git-difftool git-bisect git-rm git-fsck git-branch git-shortlog git-get-tar-commit-id git-bundle git-show git-help git-checkout git-stash git-instaweb git-cherry-pick git-status git-merge-tree git-citool git-submodule git-rerere git-clean git-tag git-rev-parse git-clone git-worktree git-show-branch git-commit gitk git-verify-commit git-describe git-config git-verify-tag git-diff git-fast-export git-whatchanged git-fetch git-fast-import gitweb git-format-patch git-filter-branch git-archimport git-gc git-mergetool git-cvsexportcommit git-grep git-pack-refs git-cvsimport git-gui git-prune git-cvsserver git-init git-reflog git-imap-send git-log git-relink git-p4 git-merge git-remote git-quiltimport git-mv git-repack git-request-pull git-notes git-replace git-send-email git-pull git-annotate git-svn git-push git-blame
plumbing
git-apply git-for-each-ref git-receive-pack git-checkout-index git-ls-files git-shell git-commit-tree git-ls-remote git-upload-archive git-hash-object git-ls-tree git-upload-pack git-index-pack git-merge-base git-check-attr git-merge-file git-name-rev git-check-ignore git-merge-index git-pack-redundant git-check-mailmap git-mktag git-rev-list git-check-ref-format git-mktree git-show-index git-column git-pack-objects git-show-ref git-credential git-prune-packed git-unpack-file git-credential-cache git-read-tree git-var git-credential-store git-symbolic-ref git-verify-pack git-fmt-merge-msg git-unpack-objects git-daemon git-interpret-trailers git-update-index git-fetch-pack git-mailinfo git-update-ref git-http-backend git-mailsplit git-write-tree git-send-pack git-merge-one-file git-cat-file git-update-server-info git-patch-id git-diff-files git-http-fetch git-sh-i18n git-diff-index git-http-push git-sh-setup git-diff-tree git-parse-remote git-stripspace
1It would seem more logical to call this --plumbing, but as VonC notes in this answer to a related question, one can view this instead as a request: “I am implementing porcelain so please give me plumbing-style output.” The flaw in this argument is that you might be implementing complex plumbing, and want to use simple plumbing to do it: now there’s no porcelain in sight, and yet, your complex plumbing passes --porcelain to some simple plumbing.