Gears are an item group from BuildCraft. They are components in most of the machines you can build in BuildCraft. The Forestry mod, being an add-on to BuildCraft, also uses these a lot.
“Stone Gears are an item from BuildCraft. They are relatively easy to build and are one steps up...Stone Gear
“  Intermediate quality gear for crafting in Build Craft. Crafting an Iron Gear requires 4...Iron Gear
“The Golden Gear is the second best gear in BuildCraft. It is used as a component in some of the...Gold Gear