Okay, what is D.I? Does and audio interface have D.I? or is that inputs?

How Does DI work?

Do you need DI to re amp? What DI/Re amping boxes are available? What would you plug the reamp box through?

DIs just convert a high impedance, line level signal to a low impedance, mic level signal for use with recording interfaces. Some interfaces will have DI inputs, others may only have XLR inputs. You can buy external DI boxes, DBX make some good ones. Try to get an active DI if you can, they usually have less signal loss than passive DIs.

DIs, when used in reamping, go between the guitar and the amp, so you only get the guitars signal (it goes before any pedals too) and you can record this through the DI's output (usually an XLR jack) Most will also have a 'through' jack with a 1/4" jack so you can run that back out to your amp. You can record the amp as well, onto a different track, but you dont have to, you can use it just for monitoring.
Reamping is just sending the signal out from your DAW/Multitrack to a reamping box (basically a reverse DI) and into an amp. Its useful because you dont need to re-record the guitar parts to use a different amp or change settings, you can tweak the amp and re-record it without even having a guitar. Once you get the settings you like, mic the amp and record it onto a new track.