The laws of classical mechanics formulated by Issaac Newton in the 17th century have been used to explain successfully the motion of objects much bigger than atomic particles. In Newtonian Mechanics, once the position and velocity of the object are known at some particular time, the motion of the object at all other times can be determined precisely and accurately.
While the prediction by classical mechanics is very accurate for big objects, they disagree with experimental results for atoms and molecules. Hence, a different set of rules for predicting the dynamics of particles, quantum mechanics, is successful at describing atomic and molecular behaviour.
Quantum Mechanics does not predict precise trajectories; in fact, knowing the precise position of an atomic particle is generally impossible in quantum mechanics, because of Heisenberg Uncertainty Principle. Instead, it only predicts the relative likelihood that a particle will be found in a range of position, much different from classical mechanics. In other words, in quantum mechanics, the motion of particles is described by probabilities.