Ley Lines are the "veins of Mystra" in the Forgotten Realms. Basically, they connect Earth Nodes. See Underdark and Champions of Ruin about Node Magic. The Geomancer also has Ley Lines as a special ability, increasing his caster level in certain terrains (nodes).
Weave: The Weave, controlled by Mystryl or one of her successors, was a way through which raw magic was accessed, tapped into and used by by casters of magic.
Shadow Weave: The Shadow Weave is a force of magic similar to the Weave. The Shadow Weave originates from Shar rather than from Mystra.
[Mystra] is believed to be the embodiment of the Weave and of magic herself, her veins the ley lines, her breath the mists and her body the pulsing, thrumming earth.
Geomancer Prestige Class, Complete Divine
Ley Lines: At 2nd level, the geomancer learns to create magical connections with a specific type of terrain.
Node Genesis (Wizard 9th Level Spell), Champions of Ruin
When you cast this spell, the arcane energies reroute local ley lines and telluric currents, precipitating the creation of a Class 1 node (you determine the type when you cast the spell). The new node is centered on and emanates from a point on the ground you touch. The rough boundary of the new node can cross any type of terrain or empty space, and can cross different types of materials. You have some leeway in determining the initial shape of your node's outer boundary—it doesn't have to be a perfect sphere.
All mentions of Ley Lines pretty much point towards Node Magic (Page 49 of Underdark)
Node magic taps the natural pools and streams of power that collect and run below the earth. This power is not visible to the unpracticed eye, but those with the appropriate sensitivity know that collection points called earth nodes are awash with power that can increase the effects of spells and produce other magical effects. Earth nodes are rare and cherished points in the Underdark, and their discoverers jealously guard the knowledge of them. However, many earth nodes that possess no natural outlet to the greater tunnel systems of the Underdark remain undiscovered, and others are simply unappreciated by the Underdark creatures that live in or near them.
Midgard Campaign Setting is all about Ley Lines. Check that book out if you haven't had the chance.