Offline sales measurement involves monitoring the correlation between online advertising and in-store sales by quantifying the increase in purchases among those exposed to a digital advertisement. Companies like Nielsen Catalina Solutions, Oracle, and Kantar Group gather data on offline purchases, including point-of-sale (POS), coupon usage, loyalty card data, and panel data. They utilize advanced database technology to correlate consumers who made offline purchases with their online cookies, establishing a connection between digital exposure and actual in-store transactions.