For an object with unchanging mass, force equals mass times acceleration. This is abbreviated as f = ma. Force is a push/pull on an object resulting from interactions with another object. Having a magnitude and a direction makes force a vector quantity.
Contact forces exist when two objects physically touch one another; action-at-a-distance forces exist when two objects exert a push/pull on each other without touching. Gravitational force is an example of an action-at-a-distance force. Summing all force vectors an object experiences gives the direction it will move. The SI unit of force is the Newton (symbolized by N). One Newton is the force required to give one kilogram an acceleration of one meter per second per second.