Submodule

• A submodule is a pointer to another git repository.
• The URL and SHA sum is kept in the parent repository.
• Submodules use the `git submodule` command.
• Submodules need special treatment; they are separate repos, and so must be checked out and updated with appropriate commands.

Subtree

• A subtree embeds another git repository entirely.
• A subtree may mix the histories of two repositories or it may be squashed.
• A squashed subtree holds only a single commit of the original repository.
• Subtrees require the `git-subtree` scripts be installed to create or update, but not use.
• A subtree is like any other file in the repo and needs no special treatment, aside from when created or updated.