The Modular Socket is the base block for all Modules in Engineer's Toolbox. Without any Modules, the Modular Socket is useless; it is an empty frame. Each side can host one Module, and each Module has a special function.
The Modular Socket does not have a GUI. However, Modules like the Inventory Interface and Tank Interface can give information, as can mods like WAILA.
Breaking a Modular Socket will give all the modules back, along with the Modular Socket itself. (In versions of the mod for Minecraft 1.6.4, the Modules were not dropped.) A right click of a side with an Engineer's Wrench will remove the corresponding Module. A sneak right click will drop the Modular Socket with all of the Modules, and configurations of those Modules, still attached to the dropped Modular Socket.
The Modular Socket comes with some storage per default: three inventory spaces, three 8,000 mB tanks, three internal redstone signals, three redstone latches and 5,000 Redstone Flux storage units.