Time, distance, and mass are the three fundamental physical quantities that most of mechanics is built upon. While there are many units that can be used to describe the magnitude of these fundamental quantities, for simplicity we will use seconds to measure time, meters to measure distance, and kilograms to measure mass.
time (s) distance (m) mass (kg)
We can build several slightly more complicated ideas out of these simple quantities. Speed is the ratio of distance to time, velocity is also the ratio of distance to time, but unlike speed which is a single number, velocity is a vector and is represented by a magnitude and a direction. You should think of velocity as speed in a particular straight line direction.
Velocity can describe an instant or an average. Instantaneous velocity is the rate at which distance is changing with respect to time at a particular instant. One must understand calculus to truly understand instantaneous velocity. The other type of velocity is average velocity. If a car drives 100 meters in 10 second, its average velocity is 10 meters per second, but it does not mean that the car traveled at a constant speed of 10 meters per second for 100 meters. It could have traveled faster during the first 50 meters and slower during the second 50 meters.
speed = distance/time = m/s
velocity = distance/time = m/s
Acceleration is the ratio of velocity to time, and like velocity it is a vector and is described by a magnitude and a direction.
acceleration = velocity/time = distance/time/time = m/s²