The JVM can be started up in either server or client mode. Server mode is slower to start but is optimised for long running programs such as servers. Client mode starts up more quickly and is optimised for programs that run for a shorter time such as desktop applications.
You can specify which version to run using the -server and -client command line options. The default depends on the specification of the machine you are running on.
How the default is chosen can be found
here [ August 04, 2008: Message edited by: Joanne Neal ]