Explanation on usage of 'td' vs 'th' within a thead

[Seth Call]

Hey there,

I know I'd personally benefit from an explanation of the usage of 'td'  
versus 'th' in a thead.

Why would someone ever put a 'td' in a thead?  Related question... why  
would put in a mix of td's and th's within a thead?

Maybe It's just a lack of creativity on my part, or a lack of  
experience with tables, but after reading the spec, I still don't  
understand why I'd use td vs a th in a thead; morever, I don't have a  
good reason why I'd mix the two in a single thead.

Thanks!

Seth