Lead is a metal resource added by IC² and generated as ore between layers 0 and 65, usually in groups of up to 6 blocks. It requires a stone pickaxe or better to mine.
These forms of lead are seen during ore processing:
These forms of lead are seen during metal shaping:
Lead is mostly useful in IC² for its radiation-shielding properties, and thus is a component of the Nuclear Reactor, Reactor Chamber, and Reactor Pressure Vessel blocks as well as the mostly blast-proof Reinforced Door. It is also used to craft basic Reactor Plating as well as the Containment Box for safe transport of radioactive materials.
Apart from these purposes, lead in dust form is a component of the Advanced RE-Battery.
Basic crafting is the same for (almost) every metal. Any pre-ingot form, except for the Tiny Pile of Lead Dust, can be smelted into an ingot.
Smelting anything but dust is a waste of resources, though - check the ore processing page to see how to maximize your output.
9 ingots craft into a block and vice versa.
Dust comes from 9 small piles, but you can't craft small piles out of a single dust like you can craft ingots out of a block.
For parts that you make using this metal, plates and item casings and so forth, there's a page for that already.