what does the time stamp at the top mail represent? actually time of sending? what about the -700 and such?

Answer 1

It represents the time that the message was recieved on Yahoo's servers. The -700 is 7 hours behind GMT - greenwich mean time.

Greenwich is the line that the official day starts on the globe - I think it's near / over England. It's a translation of the recieved time to your time zone - because the servers may not be in your time zone.