The subject is the text you see listed in your inbox for a message. I don't know how to describe this any better. When you compose a message you tend to enter the subject immediately after you enter the e-mail address or addresses you're sending to. The analog to the subject field here is "Hotmail sweep rules."
The body is the message text itself. The analog here is what you're reading at this moment.
The header is information that is used by the e-mail programs themselves to route the message from point to point and you generally don't see it unless you specifically elect to display it in your e-mail interface. The number of fields in the header is extensive and far more complex than I care to delve into right now (mostly because I don't remember a lot of them myself).
You will generally use the "From", "Subject", and "Body" fields when you set up filters for virtually any purpose.
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1803, Build 17134
In a modern society where everyone thinks their opinion deserves to be heard nothing annoys me more than individuals who mistake their personal preferences for fact.
~ Commenter TheCruyffGurn on the The Guardian website, 8/13/2014