Wotcher...
There is indeed a program called Memory Map, see
http://www.memory-map.co.uk/ for the official site. Essentially a computerised version of the OS maps you find in bigger bookshops, etc, so it shows all the different roads, rights of way, landmarks, etc.
The overlays come in, because it is possible to make up a route within Memory Map, or a list of key points. This can then be saved, and given to other people, or you can import things that others have done. They show on top of whichever map you're viewing (like if somebody draws a route on a sheet of overhead projector acetate, and you put that on top of your paper map) and are small files which are easy to share and switch between. If you get hold of the software, it can be put onto a PDA (like the iPAQ from HP, or the XDA which 02 used to sell, etc) for use out and about, or if it's on a laptop you could fit a relatively cheap GPS receiver. This means you can then use the device as a sat-nav, but for offroading, rather than just the roads found in TomTom units, etc. You can even get the software to create a track of your driving, if you know where you're going and want to share it.