Honestly, if you don't know the answer to this question, I dunno why you're even reading our website, but this might be a good link to send that relative of yours who just happens to be curious. It's not even an overly technical explanation, so maybe you won't find them later on huddled in a corner with their head between their knees sobbing...

"Furthermore, you must choose a specific type of processor; for example, an AMD Athlon 64 or Pentium 4. Therefore, you must choose which CPU you want before you can choose your motherboard."