I'm trying to set up an SPF record for my domain. I've read the RFC, been to Wikipedia, openspf.org, etc. but I'm still confused.

What is the difference between 'a' and 'mx'? When do I use 'mx: rather than 'a:' or 'include:'?

What's the point of '?all' (Neutral result) except for debugging? Why do so many people use '~all' (Softfail) instead of '-all' ?

I've been to the Wizard (http://old.openspf.org/wizard.html) but I don't understand the questions.