Actually, ModGod had it partly right.
A motorway (the M1, M5, M6, etc.) is usually three lanes (triple carriageway) although some motorways, e.g. the M11 and the northern half of the M42, are only dual carriageway (i.e., two lanes each way). Motorway rules in Britain are pretty much the same as freeway/interstate rules in the States. The speed limit isn't really enforced on most of the motorways and people tend to travel on them at between 70 and 100 mph. If you're caught doing the ton, however, it's instance loss-of-licence territory. As Jo implied, you are not supposed to travel faster than the traffic in the lane to your right, i.e. no undertaking in the left-hand lanes. Yeah, right.
Then you get the "A" roads which have motorway regulations, for instance the A1M. Confused? You betcha.
"A" roads can be single carriageway or dual carriageway and even triple carriageway in some cases. I use the A14 from Kettering to the M1 every morning, which is dual carriageway. I use the A509 to get from Wellybro to the A14 at Kettering, and it's single carriageway (and a bastard of a road due to the fact that there are a lot of people using it who believe that the speed limit is between 30 mph and 40 mph rather than 60 mph ... I do wish I had James Bond's Aston Martin sometimes, I really do.
"B" roads are everything that aren't "A" roads or motorways. Some are not bad, but some are "Z" roads in my book - really bad. Brits are pretty bad drivers on the whole and travel on narrow country lanes as if they owned the road. I don't know how many times I've had to react quickly to avoid some twat in a Jag doing 80mph on a sunken lane or hedged lane with absolutely nowhere to go.
The Brits talk about the M1 or the A14. If you said "I take A14 to M1", you'd get some pretty strange looks ...
