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The Java HotSpot Client Virtual Machine serves as a replacement for both the "classic" virtual machine and the Just-in-time (JIT) compilers used by previous versions of the Java 2 SDK to offer improved runtime performance for applications and applets. The Java HotSpot Client VM has been specially tuned to reduce application start-up time and memory footprint, making it particulary well suited for client environments.
On all platforms, the Java application launcher (the java utility) will use Java HotSpot Client VM by default when launching applications.

The Java HotSpot Server VM is similar to the Java HotSpot Client VM, except that it has been specially tuned to maximize peak operating speed. It is intended for running long-running server applications, for which having the fastest possible operating speed is generally more important than having the fastest possible start-up time.

These two systems are different binaries. They are essentially two different compilers (JITs) interfacing to the same runtime system. The client system is optimal for applications which need fast startup times or small footprints, the server system is optimal for applications where the overall performance is most important. In general the client system is better suited for interactive applications such as GUIs. Some of the other differences include the compilation policy,heap defaults, and inlining policy.


http://java.sun.com/j2se/1.3/docs/guide/performance/hotspot.html
http://java.sun.com/docs/hotspot/HotSpotFAQ.html
http://java.sun.com/products/hotspot/docs/general/hs2.html

What's different between HotSpot Client and HotSpot Server VM