This is an "A" Series Filter Holder with 52mm Ring from Cokin. The system consists of a rectangular plastic filter holder with three slots (one for Cokin rotating filters and two for their rectangular ones) and the adapter ring that screws into front thread of your lens. The "A" series is designed for standard lenses with diameters up to 62mm and is recommended for focal lengths of 35mm and longer.
"A" Series Filter Holder with 52mm Ring
"A" Series Filter Holder
52mm Adapter Ring