One: The power supply is different
The relay 24vac is powered by alternating current, and the 24vdc must be powered by direct current. The iron core of the DC relay is made of a whole piece of soft iron. If the AC power is forcibly connected, the entire iron core will continue to heat up and burn red, and eventually the relay will burn out.

Two: different application scenarios
The relay 24vdc is mainly used in equipment or electronic products with DC components, such as DC water pumps and air pumps in the scene. The relay 24vac is AC power generation, so it is generally used in some electrical appliances with AC components, such as small AC transformers.
Three: the number of turns of the coil is different
Because the internal resistance (resistance) of the 24vdc relay is too large when it is energized, it needs to be solved by a thin coil with many turns, while the 24vac relay is powered by alternating current, so the number of turns should be less than 24vdc.
Generally speaking, both 24vac and 24vdc relays have their specific properties, so please do not mix them at will to avoid unnecessary harm.