If you install Ubuntu from the live cd or alternate install cd, the ‘generic’ kernel would be installed by default. The ‘generic’ kernel focuses on i586 and i686 CPU architectures. But if you have a newer CPU, like Pentium 4 HT, Dual Core or Core2Duo, you might want to install the ‘server’ kernel to improve your PC performance. The ‘server’ kernel focuses on i686 architectures and posses some unique advantages over ‘generic’ kernel in case of task management.
Note that you should not change the default kernel if you don’t have a CPU that belong in the families stated above. And also be aware of the fact that kernel changing might cause big problems in your computer [such as making the system unstable!].