As usual, KernelNewbies provides a readable changelog that we can all understand. They sum up the most important changes in 2.6.25 as follows:
"Summary: 2.6.25 includes support of a new architecture (MN10300/AM33) and the widely used Orion SoCs, a new interface for more accurate measurement of process memory usage, a 'memory resource controller' for controlling the memory usage of groups of processes, realtime group scheduling, a tool for measuring high latencies called latencytop, ACPI thermal regulation, timer event notifications through file descriptors, an alternative MAC security framework called SMACK, a ext4 update, BRK and PIE-executable address space randomization, RCU preemption support, FIFO spinlocks in x86, EFI support in x86-64, a new network protocol called CAN, initial ATI r500 DRI/DRM support, improved device support and many other small improvements."
Head on over there for a much more detailed list of all the new features. If you're really die-hard, you can check the original, unformatted changelog too. As always, this release will find its way to your distribution of choice, but if you cannot wait, you can download the sources from Kernel.org and build it yourself.