This is a fairly large topic but the essentials to know is understand the power supply wattage's and voltages:

☛ Have an understanding of motherboard and CPU socket types.

☛ Understand different RAM types and speeds.

☛ Understand motherboard slot types (AGP, PCI, PCI-E etc..)

☛ Understand hard drive types such as IDE and SATA and understand hard drive jumpers and their configurations.

With these skills, you need to be able to assemble a computer. I am not just talking about putting one together, but building a good one by knowing their part speeds, compatibility and possible bottlenecks.