Data transmission protocols are sets of rules that determine how data is sent and received over a network. They ensure that information is transmitted accurately and efficiently between devices, such as computers and servers. Common protocols include TCP/IP, which is fundamental for internet communication, and HTTP, used for transferring web pages. These protocols define aspects like data format, error checking, and how connections are established and terminated. By following these guidelines, devices can communicate seamlessly, allowing for activities like browsing the web, sending emails, and streaming videos without issues.