Software is the "brain" of computers and its development is often ingenious and time-consuming. Software is usually not sold, but licensed: the "buyer" or correctly, licensee, pays for a certain use (the same as for music CDs: one buys the medium, the glitter disc, not the music but one is allowed to listen to it). For example, Microsoft "sells" licenses for private users that allow installation  of the software on one computer and a to make a back-up. To make more copies is unlawful and is copyright infringement (although practically often possible). This type of software is called proprietary software. Software is protected by copyright, in the US also by patents. The discussion of protecting softare by patents is ongoing in Europe.

Open source software  (OSS) is computer software whose source code is available to the public under a  license that permits users to use, change, and improve the software, and to redistribute it in modified or unmodified form. It is often developed in a public, collaborative manner (Source: Wikipedia). That means, you are subject to certain rules also with OSS, albeit different ones than those applicable to proprietary software, depending on the license type.

What is Open Source Software ?
WIKIPEDIA article on Open source
Creative Commons
Legal issues in OSS (Report of the Legal-IST project, mainly geared at lawyers)
Norwegian OS articles:
To nyttige lenker
Snubler nye GPL versjon 3 i egne ben?

A free  on-line course of www.netgeners.net: Copyleft vs. Copyright
Intellectual Property Rights  in USA

Related legal Wiki articles:
Licensing
Copyright
Copyleft
Intellectual Property Rights