MELPA is an ELPA-compatible package repository built automatically on our server from the upstream source code using simple recipes. (Think of it as a server-side version of el-get, or even homebrew.)
Library authors do not need to upload new versions of their packages to MELPA, since they are built automatically. The MELPA maintainers plan to add support for automatic builds of stable version packages triggered by upstream version tags.
Currently supported backends are git, svn, bzr, hg, darcs and emacswiki. The collaboration model for MELPA is like that of the Homebrew package manager on OS X – to add new recipes, simply file a pull request on Github.
Melpa uses a recipe system similar to el-get. Recipes are stored on a git-hub repository which tells the updated package.el how to download each package. The source of the package might be from git, svn, darcs or this wiki.