In physics a point particle is defined as a body whose dimensions are negligible compared to the phenomenon under study. For example a planet can be considered a point particle in a celestial mechanics problem, an atom in a statistical mechanics problem and so on.
In general a point particle is only characterized by its three spatial coordinates, its velocity and its mass. This means that the schematization of a body as a point particle is equivalent to neglect the existence of its internal degrees of freedom: a point particle can not store energy by rotating on itself, heating or compressing elastically. All these phenomena, in fact, to be described need a more detailed modeling of the body: always referring to a concrete example, a planet can be treated as a rigid body, rather than as a point particle, if we are interested in its rotation. The usefulness of the concept of point particle is to be able to associate a geometric point to the body and then to operate in Cartesian space with the methods of analytical geometry.