what is a 'HYRDO' transmission?
 

I know what a cable tranny is, i presume it's the external linkage from the shifter to the tranny. So what's a hydro?

hydro means that the clutch is disangaged/engaged by a hydraulic system rather than a cable

with a hydrauic tranny there is a master cylinder that attaches to the clutch pedal, which then sends brake fluid through the hydraulic lines to the slave cylinder. The slave cylinder then actuates the clutch release fork.