Well as you may know, there are a number of communication protocols. Each one is used for a different reason.
To answer that question you probably will have to mention that it is "a set of rules that is governed between two computers"; used to determine how the data will be transferred, checked and received.
Some examples:
HTTP: This one is used to publish and retrieve pages on the internet
FTP: This stands for File Transfer Protocol and as the name suggests it is used to transfer data across the internet or through a network. You use FTP to upload your website pages to the web server (for example).
There are also some email protocols like
POP and
SMTP, POP is used to retrieve email from an email server and SMTP is used to transfer email messages to the web server.
These are only a few examples of protocols, you can find many more.
To answer your question about Error Checking; when two computers / servers try to connect to one another they usually use the same protocol to do so. The first step in the connection is "Handshaking". This is where specific information is give to the other computer and vice versa. This information can include the type of error checking that will be used; CRC, Check Sum, Parity Bit etc, as well as information such as what is to be transferred and anything else relevant to the connection.
