The question is so ambiguous it can't be answered. It's like asking "what is in a photograph?"
A PCB (assuming you mean a Printed Circuit Board) is a final product, it could be absolutely anything. I have PCBs here ranging from microwave video transmitters to LED flashers and IC programmers, not to mention the ones inside my computers, mobile phone, landline phone, TV, ham radio equipment and wristwatch. Even those of us with long experience can't always tell what a PCB is at first glance, we have to look for clues, the kinds of components and connectors, whether it has small or power rated components on it, if it seems to have tuned circuits on it and so on. Even if we work out what it does, it could take weeks to reverse engineer it back to a diagram and that doesn't even start to unravel what any firmware might do.
Please be far more specific, which PCB are you trying to read and what do you expect to learn from it?
Brian.