Ratios are given in how many times the whorl on the flyer (or bobbin, if an Irish tension) turns for every one turn of the drive (large) wheel.
For thicker yarns, or making yarns from fleece/roving with a long staple length, you will want less twist in your yarn, so you choose a low ratio.
For thin yarns, or yarns made with short staple lengths (like cotton), you'll want more twists in your yarn and want to use a higher ratio.
All this means is you can treadle at the same speed and by changing the whorl ratio, can put a different amount of twist in your fiber. Alternately, you can leave the drive band on the same whorl and slow down or hurry up your treadling speed.
Single Drive (SD) Scotch tension means your drive band goes around your drive (large) wheel, then around the flyer (U shaped thing). A separate brake band goes round the bobbin to slow it down so the yarn can wind on to the bobbin. This is called a 'flyer lead' since the drive band controls the speed of the flyer.
Single Drive (SD) Irish tension means your drive band goes around your drive wheel, then around the bobbin. A separate brake band goes round the flyer to slow it down so the yarn can wind on to the bobbin. This is called a 'bobbin lead' since the drive band controls the speed of the bobbin.
A Double Drive (DD) means the drive band goes around the drive wheel, around the flyer, back around the drive wheel and then around the bobbin. There is no brake band.
Most wheels are either Scotch tension, Irish tension or DD. My first wheel, a 2002 Ashford Traveller actually has all three. When I started spinning on a wheel, I found the DD the least complicated and easiest to use.
I believe Babe wheels are Irish tension. Some folks have problems with Babes 'walking' during use because it is so light weight. The solution is to put sand in the legs.
Since you travel, another option for you would be the Hitchhiker