A protocol stack is a part of an operating system (OS) or device that is used to communicate with other things.

Computers communicate in many ways. A mobile phone contains several protocol stacks. There are protocol stacks in a mobile phone for handling voice calls, for Bluetooth, for Wi-Fi, and for USB. There is a protocol stack used when reading a webpage on Wikipedia.

Sometimes the protocol stacks work together. An example of this is when a person uses a Bluetooth headset for a voice call. One stack is used for the phone to communicate with the headset. Another stack is used to make the voice call. The Bluetooth headset or earbud has its own bluetooth protocol stack.

The protocol stacks are used to send and receive information.

Each layer of the protocol stack handles a different part of the communication. There is a protocol stack with many different layers that are used to send and receive data. When two things communicate with each other in a standard manner, this is known as a protocol.

Some layers within the protocol stack will work on reliability. They will send a message again if the first message was lost in communication.

Some layers will provide compatibility. They will let data to be communicated in different ways, like over Wi-Fi or over a network wire, but the layers above don't need to know what way is being used. And whether wifi or wired - those layers don't need to know if it is carrying text in a wikipedia page, or perhaps the data in a picture. They let different types of data to be communicated in a standard manner.

Layers often handle a series of chunks of data. Some layers split the data into smaller chunks when sending it. This is called fragmentation. When the data is received, another layer puts the smaller chunks back together to form larger chunks or media streams. This is called reassembly.

Some layers may encrypt the data when it is sent. This is done so it cannot be read. A layer at the receiving side will remove the encryption, known as decryption. Secret keys are used to encrypt and decrypt the data.

The OSI model is a common way to think about protocol stacks.