Using Notebook is older common use term. The term laptop has in recent become more popular.
http://en.wikipedia.org/wiki/Laptop. But notebook was more common in the 80 & 90's IMO.
PC is generally taken to mean IBM compatible PC. As that was convenient and more meaningful term to group all compatible computers. This would exclude Mac's, Amiga's. which weren't compatible. However Apple/IBM/Motorola came up with PowerPC and started using that, and then now we have Apple using IBM compatible hardware. So these days the term has been blurred. It is used quite a lot in Apples marketing I suspect deliberately to put an Apple computer right up there in people consciousness as a valid alternative to a PC (IBM compatible). Which is a different argument, but in many ways any computer which is used for personal computing, even a PDA is a PC.
Perhaps someone with limited IT exposure wouldn't be familar with all this though.
Light and small and inexpensive are unlikely combinations. Usually light and small computers are very expensive. You are probably best looking at a compromise. Which is a 14" laptop of low spec. One option I'd suggest looking at is the Dell 640m, and see if you can pick up a deal on one on the Dell Outlet. I've got them before for €600. For the price they have great battery life. If you get one with a 9 cell battery they can have 7-8hrs battery life. Battery life being important for a travelling salesperson. Note that a DualCore or Core 2 Duo has bettery life than a Celeron, and the smaller the screen, and the slower speed of the laptop the better battery life usually.