In terms of source control, you're "downstream" when you copy (clone, checkout, etc) from a repository. Information flowed "downstream" to you.

When you make changes, you usually want to send them back "upstream" so they make it into that repository so that everyone pulling from the same source is working with all the same changes. This is mostly a social issue of how everyone can coordinate their work rather than a technical requirement of source control. You want to get your changes into the main project so you're not tracking divergent lines of development.

Sometimes you'll read about package or release managers (the people, not the tool) talking about submitting changes to "upstream". That usually means they had to adjust the original sources so they could create a package for their system. They don't want to keep making those changes, so if they send them "upstream" to the original source, they shouldn't have to deal with the same issue in the next release.

http://stackoverflow.com/questions/2739376/definition-of-downstream-and-upstream/2749166#2749166

What is the difference between origin and upstream in github?

When a git branch -a command is done, some branches have a prefix of origin(remotes/origin/..) while others have a prefix of upstream (remotes/upstream/..).
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This should be understood in the context of GitHub forks (where you fork a GitHub repo at GitHub before cloning that fork locally)

• upstream generally refers to the original repo that you have forked
  (see also "Definition of “downstream” and “upstream”" for more on upstream term)
• origin is your fork: your own repo on GitHub, clone of the original repo of GitHub

From the GitHub page:

When a repo is cloned, it has a default remote called origin that points to your fork on GitHub, not the original repo it was forked from.
To keep track of the original repo, you need to add another remote named upstream

git remote add upstream git://github.com/user/repo.git

You will use upstream to fetch from the original repo (in order to keep your local copy in sync with the project you want to contribute to).

You will use origin to pull and push since you can contribute to your own repo.

You will contribute back to the upstream repo by making a pull request.

http://my.oschina.net/uniquejava/blog/481625