a non-modular PS is one in which all the power connectors come from the same small hole on the side of the unit, none of which can be detached from the PS. a modular PS on the other hand has multiple plugs in the side, allowing you to attach only the cords you absolutely need, cutting down on clutter and increasing airflow inside the case. i've also seen some PS's that are semi-modular, where the connectors that are essential in just about any situation (such as the typical 20/24 pin main motherboard power connector, the 4/8 pin CPU connector, etc.) are fixed to the PS and not removable, while the non-essential lines and connectors (such as PCIe, SATA, and molex power connectors) remain modular (removable).