Velocity is a very formal, technical equivalent of speed. It is especially used when talking about science (for example in physics). There is no difference in the basic meaning of the two

Technically speaking, 'velocity' is a vector and 'speed' is a scalar. This means that 'speed' is only a magnitude and 'velocity' is a magnitude AND a direction. People often use them interchangeably colloquially, with 'speed' being more common.