Various ways to transfer data between two repositories (possibly over the network):

generally

• documentation about the URL formats
• man pages
  • git-bundle
  • git-clone
  • git-fetch
  • git-ls-remote
  • git-pull = fetch + merge
  • git-push
  • git-remote
  • less often used
    • git-http-fetch
    • git-http-push
    • GIT-parse-remote
    • git-receive-pack
    • git-remote-helpers
    • git-shell
    • git-send-pack
    • git-upload-archive
    • git-upload-pack
• Git book [1] [2]
• Pro Git book — chapter 4 "Git on the server"

read-only servers

• git daemon — bundled with git
• any HTTP server

servers that allow updates, via git://

• git-daemon has the option --enable=receive-pack, however, there is NO ACCESS CONTROL on this — anyone with network access can modify your repo

servers that allow updates, via ssh://

• Gitolite [apt-get install gitolite]
• Gitosis [apt-get install gitosis]
• more
• more

servers that allow updates, via http://

• git-http-backend (available since Git v1.6.6, released in January 2010)
  • pretty good writeup here
  • more info: [1] [2] [3] [3] [more]
  • Ubuntu-specific setup: [1], more
• SCM-Manager