Performance monitoring is a critical aspect of vSphere administration. This article introduces you the basic concepts and terminologies in vSphere performance management, for example, performance counters, performance metrics, real time vs historical statistics, etc. Much of the content is based on my book VMware VI and vSphere SDK by Prentice Hall.
Once you understand these basics, the related tools and APIs should be relatively easy. If you are already familiar with vSphere Client performance monitoring or esxtop, they help as well.
Performance Statistics (PowerCLI)
Performance Statistics (Perl SDK)