The main difference between solder paste and flux is that solder paste is a semi-solid form of soldering material that contains both the metals needed for the connection and a flux, while flux is a substance used to reduce the surface tension of molten metal, which allows it to flow more easily.
Solder paste is often used in surface-mount technology (SMT) because it can be applied directly to the pads on a circuit board. Flux, on the other hand, is often used in through-hole technology (THT), as it helps to clean the metal surfaces and prevent oxidation.