I have heard a lot about packets in networking. What do they look like? How are they transmitted through a medium like air?
To transmit data you need a media (copper or optical fiber, for example) and each media has its own set of features than make it usable for data transmission.
For example copper is a good electrical transmitter and it's cheap (compared to other elements as silver or gold). Then, the best way to use it is to shape it as a cable and transmit electricity through it.
Now, suppose you need to transmit a jpeg image. The telecommunication protocols divide the whole file in chunks that can be transmitted and verified easily so the transmission is reliable. Those chunks are packets, and inside you will find a stream of bits. Each bit can be represented as a change in voltage of a electrical current.
The following is a simplification of bits traveling the cable:
When we use antennas as wifi or mobile (GPRS, 3G, 4G) we use electromagnetic waves. All around us there are electromagnetic waves (most of them are invisible, but there are other waves that we can see: the light is formed of electromagnetic waves, and the different colors that our eyes can perceive are different frequencies of the waves. Check the following chart):
So, the trick is to choose a range of frequencies and use waves in that range, that is invisble to the human eye. Electromagnetic waves don't depend on air, they can travel on vacuum allowing us to communicate with spaceships, satellites, etc.
How do they look?
Remember how in the copper cable a bit 1 was a high voltage and a bit 0 was a low voltage?
In electromagnetic waves we don't have voltages but we can play with frequencies. A bit 1 can be a frequency and a bit 0 can be a lower frequency than that of the former.
So, sending packets through electromagnetic waves could look like this (this is an oversimplified example):