opam
opam is a source-based package manager
...). This is all you need for installing and using opam, but if you want to use the opam-lib (to work on opam-related tools), you need to link it to installed libraries. It's easier to already have a working opam installation in this case, so you can do it as a second step. If you install on your system (without changing the prefix), you will need to install as root (sudo). As sudo do not propagate environment variables, there wil be some errors.