How to understand SGA?

For SGA, from wikipedia <https://en.wikipedia.org/wiki/System_Global_Area>, its definition is:

the System Global Area (SGA) forms the part of the system memory (RAM) shared by all the processes belonging to a single Oracle database instance.

But from Oracle 12c For Dummies
<http://www.amazon.com/Oracle-12c-Dummies-Chris-Ruel/dp/1118745310>, its definition is:
The System Global Area (SGA) is a group of shared memory structures. It contains things like data and SQL. It is shared between Oracle background processes and server processes.

So what is the meaning of "server processes" here? Which definition is more accurate? How to understand SGA?

Thanks very much in advance!

Best Regards
Nan Xiao

--
http://www.freelists.org/webpage/oracle-l