"Dob" just means Dobsonian....it refers to the type of mount the telescope sits on. It was originally designed by a guy named John Dobson, in the US, many years ago. 6" and 8" just mean 6 and 8 inch...the size of the main mirrors of the scopes. A 6" scope will see stars down to about 12-12.6 magnitude, which means it can see objects upto 275 times dimmer than you can see them with your own eyes. A scope of that size will give you plenty to look at...all the planets (not Pluto though...too dim), every star you can see and then some, pretty much all the objects in the catalogues like the NGC and IC catalogues (that's nebulae, star clusters, galaxies etc). You can see a lot with a 6" scope, but the bigger the mirror your scope has, the more objects you'll be able to see and the brighter any object will appear. You'll also see more detail in any given object because a larger mirror gathers more light and has a higher resolving (seeing) power than a smaller scope.
So, in light of this, a 6" scope is good...an 8" is even better
What you really need to consider is how much money are you willing to spend. Then it's a case of getting the biggest scope you can afford. With some of the trimmings, of course, like eyepieces, barlow lenses etc.
Hope that has helped
