User's Guide Book: Overview --> Free (Libre) and Open Source Software (FLOSS)

First of all, I think we should use EU terminology, also - the main definitions of lead organisations (FSF, OSI, etc.).

Free/Libre/Open Source Software, the term most commonly used when talking about either Free (Libre) Software or Open Source Software.
Open Source Software is plain software, just like you know already. When in operation, you won't notice that the software is actually Open Source. The difference with its counterpart (proprietary software, also known as closed source software or CSS) is the way in which it is licensed.
Source code
The human-understandable texts that programmers type to write applications.

What is Open Source Software? -

The Many Aspects of Open Source -


More info on terminology: Glossary.