Of course, spacecrafts don't send telemetry data to us in a format we can understand. This is important because ground stations and processing centers around the world can use similar equipment. Its sort of like all the European countries using the same currency it just makes things simpler. Header contains the routing information Application Data Field contains the contents.
Packets are variable in length.
Transfer frames are what the tracking station sees coming down from the spacecraft. It is also helpful on the ground to further separate different types of data, such as real-time telemetry, stored housekeeping data, stored science data, etc. By setting a flag in the header of the transfer frame, the ground can sort them separately. The MOC uses ITOS software to process incoming telemetry. At the MOC, the frames are processed, and the packets are pulled out of each frame.