What is the format of the fix protocol message?

Could you please elaborate on the structure and format of the fix protocol message? I'm particularly interested in understanding the various fields that comprise this message, their data types, and the order in which they are typically arranged. Additionally, I'd like to know if there are any specific rules or guidelines that govern the formatting of these messages, and if there are any common pitfalls or challenges that developers should be aware of when working with the fix protocol.