Ley Lines were originally just an observation that a number of British and European landmarks occurred in relatively straight lines: draw a line between say a church and a barrow and extend those lines outwards and you'd likely hit other 'significant' landmarks such as a crossroads, ford, burial ground, mounds, marking stones, etc. Trade routes ('ley' is an Anglo-Saxon word meaning 'cleared strip of ground'; the original idea is that they were surveyors lines) or old highways usually make up some of those, because the straightest distance between two points, etc etc. Now since in Britain you can't walk 10 feet without tripping over a haunting, a fairy mound, a place where a saint worked a miracle, etc the idea came about that these formed conjunctions or 'rivers' of power. People
love to find patterns in things and like as not even the most tenuous pattern will acquire a significance beyond it's actual meaning.
If you go poking around the New Age section, you'll also find references to naga lines and dragon lines in India and China respectively, proving that anything worth doing is worth over-doing but many peoples have beliefs regarding straight lines. The Hmong, for instance, beleive that spirits can only move in straight lines. The lines on the plain of Nazca are sometimes brought into this. There will sometimes be a correlation drawn between features and Earth's magnetic field (not a bad correlation, since any culture who invents the compass usually instantly orients their stuff around it for convenience's sake).
Here's an example of a ley line map. Note that a lot of primary leys are named.
The idea has evolved into an idea that magic flows like rivers do, pooling in these lines between areas of mystical significance. Places where lines cross are even more powerful. Most depictions of ley lines have them completely straight, a few have them curve around. Most depictions have them mostly random among themselves, while in others the lines themselves form patterns (such as pentacles, etc) at or around particularly sensitive places.
Usually ley lines in fiction are used as a power source. Sometimes magic only works near them, but other times they serve as a power booster to existing spells or effects. They can feed power into already ongoing effects to boost them. Into the Darkness, Turtledove's magical WW2 series, has them used as a transport mechanism.
Obviously to use them well, you need to first map them out. This may well explain why the mages need to use an abandoned tower out in the middle of a bog for the big ritual, because that tower sits at a ley line crossroad. Assign some mechanical benefit to tapping a line (maybe all spells are cast at +1 caster level in the vicinity of a line) or a nexus point (spell penetration increases by 5, say).