Steady flow of fluid is a condition where the function of fluid property is time independent. This can be said as a condition of a fluid which remains same over a period of time. Unsteady flow of fluid is the condition where the function of the fluid properties is time dependent, i.e., the properties of the fluid change with respect to time. Such unsteady flows can also be called as transient flows.
Steady flow does not imply that velocity and acceleration are constant .For example consider fluid flow in a tapered pipe, which may be steady, but the velocity and acceleration need not be constant. Some examples of steady flow devices include pipes, nozzles, diffusers, and pumps.
True steady flow is present only in laminar flow where as in turbulent flow, there are continual fluctuations in velocity and pressure at every point, so flow might not be steady in nature.
Truly uniform flow is one in which the velocity is the same at a given instant at every point in the fluid. A non-uniform flow is one in which velocity is not constant at a given instant.