Tourers look like normal road bikes, but tend to have lots of fairing, comfortable and relaxed riding positions, loads of luggage space, massive fuel tanks and groovy bits to enhance the 200+ mile tank range. They tend to have a bit of an Old Man image.
Typical bikes include: Pan European ST1100, BMWs, Goldwing, Deauville, Pretty much anything ridden by the Police.....
Sports-Tourers are halfway between Tour and Sport bikes, usually performance focussed with all the above features, but to a lesser extent. Often ridden by men who will one day become Old Men.
Typical bikes include: Sprint ST, FJR1300, Blackbird
Cruisers are long, low, retro-styled bikes with a laid back, feet forward riding position, leather & chrome everywhere, big thumping V-Twin engines, LOUD and bassy exhausts, tassles, more chrome and serious pussy-wetting imagery. Often ridden by blokes with big beards, bandannas and sunglasses.
Typical Bikes: Most Harley Davidsons, Virago, Dragstar, Intruder, Marauder, Shadow.
I know of 125 Cruisers, but I doubt proper Tourers come in 125...
I'd heartily recomend either to aspire to.
As for a 125 - A decent 125 cruiser will set you back around £1,500 - £2,000. I'd go with the Dragstar or maybe the Shadow. Viragos are built for small women (because they look so sexy on one) and the rest are kinda crap.
But for just needing a bike until you pass your test, anything that works, is comfortable and is affordable will be all you need. As the Bill Hicks fan below says;