Order Blocks are small "pauses" in the market before major moves and reversals. They also often form mid-move while price "catches its breath before continuing in the original direction.
Often, Order Blocks contain within them (on lower time frames) Supply and Demand clusters and frequently act as a form of psychological Support And Resistance as these are areas of consolidation at which time important historical orders are filled.
Whereas Order Blocks are often just ONE candle in the middle of a move, Supply and Demand clusters/zones are often rectangular in shape and usually form over a number of candles.