Distance is the actual length of the path the particle has traveled. It is always positive or 0 if the particle is stationary. Distance is a scalar quantity and it does not depend on direction of travel. For finding the distance we have to trace along its path.
Speed is the rate of change of distance. To get the distance traveled, we use the speed and time.
Displacement of a particle is the line joining the initial position and the current position at current time t. It is a vector quantity. It has a magnitude and a direction from initial point to the final point. magnitude of displacement vector can be negative or positive or zero.
To find the displacement, we use the velocity vector and time. Rate of change of displacement is the velocity.
If a person starts from A and goes to B and then C, the distance traveled is 2 meters. But the displacement is the length of AC, it is smaller than 2 meters. Also, the direction is from A to C.
In the other diagram, if a person walks from D, to E to F and then to D, the final displacement is zero. but the distance traveled is 3 meters.
Uniform motion : if a body travels with a constant speed then it is in uniform motion. If a body covers equal distances in equal periods of time, then it is in uniform motion. Both definitions are same.