Submodule

sub-module creates a link to a repository (no file, no commit, no history is saved within the main project repository)

Subtree

sub-tree integrates a subprojects repository within a subdirectory, optionally including the subproject’s entire history into the main project

Usage: Include the source code of a library as a subdirectory of your application.

Unlike submodules, subtrees do not:

• need any special constructions (like .gitmodules files or gitlinks) be present in your repository,
• force end-users of your repository to do anything special or to understand how subtrees work.