Purpose of - (hypen) in script or command line

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# 1  
Old 07-25-2011
Bug Purpose of - (hypen) in script or command line

I am new for unix and I am following ABS guide. What is the purpose of - (hypen ) in the below command and What it will do in this?. Can anyone explain it in detail. Rest of the things in the below command I understood somewhat.

(cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xpvf -)
# 2  
Old 07-25-2011
It's common (and POSIX) convention for stdin/stdout:
Utility Conventions
"For utilities that use operands to represent files to be opened for either reading or writing, the '-' operand should be used only to mean standard input (or standard output when it is clear from context that an output file is being specified)."
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