Both help protect tile installations, but their functions and installation requirements differ.
An uncoupling membrane allows relative movement between the tile covering and its substrate, reducing the transfer of stresses that can crack tile or grout.
A crack-isolation membrane is designed to reduce the transfer of movement at substrate cracks into the tile layer. It may be a sheet or an applied coating; its permitted uses depend on the product and installation system.
Do not assume that one product automatically satisfies the requirements for the other. Follow the specified assembly and the membrane manufacturer’s instructions. A crack-isolation membrane does not eliminate the need for movement joints in the tilework.
Sources: Tile Council of North America guidance on crack-isolation membranes and Schluter technical director’s explanation of uncoupling membranes, PDF pages 1–3.