Any substance that flows is in fact fluid. All gaseous materials and some liquid materials are considered fluid.
Liquid is a state of matter that has a defined volume and a shape that is based on the container it is held in.
(Essentially, the difference between fluid and liquid is the viscosity, which measures thickness. Fluids have higher viscosity than liquids. An example would be motor oil which has a higher viscosity than water.
Fluids can also be both incompressible and compressible, but liquids can only be incompressible)