TCP protocol

TCP (Transmission Control Protocol) is a protocol responsible for data transport and along with IP protocol is a basis for the TCP/IP model (about which you'll learn here).

It works in OSI model fourth layer - the transport layer. You can read about OSI here.

It is connection-oriented, i.e. both participating machines need to communicate and confirm transmission for data to be transported. The initialization of communication in TCP is called three-way handshake.

SYN (standing for Synchronise) is sent by the client machine and initialises communication. SYN-ACK is a confirmation of communication sent by the receiver, ACK standing for Acknowledgment. Three-way handshake is ended with ACK command, being sent by the initiator.

Three-way handshake abuses can be used in DDoS attacks, during which victims are flooded with SYN or ACK commands. If you want to learn more about DDoS attacks, read this article.

TCP ensures high quality of data transmitted, protection from data loss and ensures it is sent in proper order. It is not responsible for data packaging or addressing - this is done by the IP protocol.

Another protocol used for data transport is UDP, which does not require mutual agreements.

If you're still not entirely sure what TCP does, compare it to a courier - he does not address or package mails, instead picks it up from senders and gives to addressees, should they agree to receive them.