In the sections following, we'll discuss each layer in detail. At the top most layer, the Application Layer where data originates from, data is passed down the stack during the communication process and passed back up the stack by the receiving end. Data is organized into packets which are like little independent units of data that have all the information it needs to get to where it's going, attached to it. It not unlike sending a letter in the mail. It's an independent envelope of information with return and destination addresses on it.
Packets are pass down the stack as a header is slapped on the data by each layer protocol.
Each layer appends their header ignorant of other headers that are already on it. It sees the packet with the headers just as one chunk of data. And as the packets travel back up the stack, each protocol strips their headers off the packets, so the data is back to its original state.