These are very informal descriptions of terms people often use when discussing open source licenses and software. Many licenses and organizations define these terms differently and/or more precisely.
Terms I
Term
Overview
Source
source code - files created by programmers using a programming language
Open Source (software)
source code that can be used, studied, changed, and redistributed
Redistribution
giving software to another party (possibly with modifications)
Fork
project started by modifying another project's source
Shared Source
source can be obtained, but with restrictions
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Terms II
Term
Overview
Free Software
term similar to 'open source'
Proprietary / Closed Source
traditional software with unobtainable source
Copyright
most open source licenses derive their power from copyright law
Copyleft
requires redistributions to be open source
Permissive
permitting most options - usually including proprietary forking