shared_buffers

I'm reading the docs for 8.4.2, section 18.4.1 Memory. I'm trying to figure
out what reasonable values for my usage would be.

The doc says:

shared_buffers (integer)

    Sets the amount of memory the database server uses for shared memory
buffers.

While circular definitions are always right, I'm kind of looking for some
information like:

"PostgreSQL uses shared memory buffers for ...."

Could someone please explain what the role of shared buffers is?

George Sexton
MH Software, Inc. - Home of Connect Daily Web Calendar
http://www.mhsoftware.com/
Voice: 303 438 9585