database files

I am investigating the possibility of using pgsql as the database in an application.  I have some unusual requirements
thatI'd like to ask you all about.  I apologize in advance if my terminology is a little "off", I'm not familiar with
pgsql(yet). 

I need to be able to move the database files, as normal user-visible files, between machines. I.e. given a database on
machineA, I want to be able to copy either a single file (ideally) or a single directory (less ideal but still ok) to,
say,a zip drive, bring it over to another machine (with pgsql also installed), start up my application and have it
accessthe copied database through pgsql. 

Is this sort of thing possible?  Is a database stored in a single file or multiple files?  Can the location of the
file(s)be controlled?  Are the files accessible and consistent while pgsql is running? I assume not all the time, but
isthere a reliable way to make them accessible (i.e. copyable) and consistent short of shutting down pgsql? 

Is the file format of the pgsql database files compatible between OS's?  E.g. could I take some database files from
Linuxand use them on Windows? 

Thanks in advance for any advice,

Gail Zacharias