A Ley-Line is a current of great and powerful arcane magic. Ley-lines encircle all of Azeroth like veins and come in many different sizes and various degrees of power. Ley-Lines can be mapped and measured by several methods. The Arcanomicon is a map of all the ley lines on Azeroth. Places where two or more ley-lines cross are known as a ley-line nexus, and are often visible in nature as places of great power. All ley-lines on Azeroth converge to a single Nexus upon which the tower of Karazhan stands.
Every culture recognizes the ley-lines in some fashion. Many ley-line scholars feel that ley-lines were a major factor in deciding which place are sacred to the worshippers of the Light and where to place moonwells. Many put great importance on the places where ley lines meet, known as a ley-line nexus. Dalaran and Silvermoon City were both constructed on massive ley-line nexus'. Despite the importance people put on the convergence of ley-lines, any ley walker will tell you that the patterns made by the ley lines are just as important. They are known as runes and are an important part of magic.
Ley lines on Wowpedia.