An open source program is a program where the source code is
freely available for developers to modify and improve. A closed
source program can only be modified by the people who have the
source code, (eg. The company that made the program).
Examples of open source programs would be: Firefox, Google
Chrome, Audacity, 7Zip, VLC Media Player, Pidgin ect.
Commercial software like Microsoft Windows, Internet Explorer,
MSN, Safari ect. is closed source.
Also large programs such as operating systems (i.e Linux, Mac OS
X) often use mixed source and/or shared source. Mixed source is
where some of the program is open source and the rest is closed
source.
Shared source is where only developers who join a special
network (usually non-free), can access the source, and only they
can modify and improve the software. The major supporter of shared
source software is Microsoft.
Also don't get confused. Free software means it can be used
without charge, it does not mean it is open source. Shareware does
not mean shared source, it means it can only be used for free with
fewer features and/or for a limited time (a free trial)