Your motherboard is like your central nervous system. It connects all your devices to work together as a unit with the CPU. Generally you don't want to get a cheap motherboard. Motherboards should be compatible with your CPU, meaning the motherboard should have the right socket to work with your CPU. If not no go. The motherboard you choose should be durable so they can last a long time and make sure you have enough SATA connections for the many hard drives, solid state drives, Blueray drives, and etc. We next move to the number of PCI slots. If you're a gamer this is important. Depending on the size of the motherboard (ATX, miniATX, etc) will correlate with the number of PCI slots you have. Most gamers and graphics heavy programmers might want two PCI slots for a second upgradable graphics card in SLI or Crossfire.
Next is the RAM (Random Access Memory). RAM are like the lungs on the human body pumping oxygen in and out. RAM constantly has electrons being pumped in and out as it loads information from the CPU. Generally 1333MHz RAM is a good speed. Anything faster than that (especially for gaming) is not going to make much of a difference. You do however want to make sure you have enough (8GB will be plenty). Lastly, handling the RAM and all the other components of the system you want to make sure you ground yourself so you dont shock the crap out of your components and short circuit everything.
Next up we have the Power Supply Unit (PSU). You need a good PSU from a good manufacturer for two reasons: energy consumed and reliability. If you cheap out on the PSU and it malfunctions for some reason it could fry all your internal components.
Storage drives holds your operating system and other media content. These days solid state drives are a hott topic since they contain no moving parts and are much faster than hard drives. The operating system should be installed here generally to have a fast system.