Demand Side Platform and Supply Side Platform work on similar technology to a great extent, but their end-users are different. In an upside-down view of the programmatic ad selling, the DSP may look like SSP and vice versa.
As DSP aims to buy the ad space, the approach is more buyer-centric, i.e., buy more ad space in less amount, while SSP, often referred to as the seller-side platform, tries to sell the ad space to the highest bidder.