What is the format of a FIX message?

Could you please elaborate on the format of a FIX message? I'm curious to know how it's structured and what kind of information it typically contains. Is it a standardized protocol, or does it vary depending on the type of transaction or the messaging system being used? Additionally, how does the format ensure the accuracy and reliability of the data being transmitted? I'm looking to gain a better understanding of this crucial aspect of financial messaging.