Originally Posted by jbnc
The software that you install in linux comes from places called repositories. There are default ones (which have completely free licenses) and there are others which have more restrictive licenses, don't come with source, or are completely proprietary.
Since you mention using Ubuntu 7 (7.04, ie the current one) you should already be set up to use the universe, multiverse and restricted repositories.
If you compile software from source, you also need compilers and suchlike software development tools. g++ is the GNU C++ compiler. If its not found, the means it has not been installed (Ubuntu, unlike most other Linux distributions, doesn't ship with compilers etc by default).