A packet consists of two kinds of data: packet information and packet "data" (or "payload"). If you think of a packet like a letter in the mail, the "control information" is like the outside of the envelope. There's an address that says where to send the letter, and a stamp that says how to send it (fast or slow). Similarly, the computer uses the "control information" in the packet to decide where, and how, to send the data. People aren't usually interested in the control information.