The object or the group of objects whose motion is being described using a model. Aand the environment influences the system only through interactions that generate forces. More generally, the part of the universe upon which attention is concentrated, thereby creating an internal and external region - the later called the environment. Systems are abstractions of reality that have structure, variables to describe their state, and structural and functional relationships among their constituents.
System and Environment
Definitions
In each physics problem, some object or objects will have their motion described by a chosen model. That object or objects then form the system that the model applies to. There will usually be other objects that influence the motion of the system, but whose motion is not modeled. Those other objects constitute the environment. The environment will influence the system only through interactions that generate forces, work or [torques] on the system.
Examples
Block on Inclined Plane
In a typical inclined plane problem where a box slides down the plane, the box would be the logical system to consider. The plane and the earth will be the environment. The environment affects the system through interactions. In this example, the earth affects the system through the force of gravity, and the plane affects the system through forces like the normal force and (possibly) friction.
Atwood's Machine
In a typical Atwood's Machine problem where two boxes are held together by a rope that passes over a massless pulley, the boxes will generally be treated as two separate systems. The relevant model (Point Particle Dynamics) is most easily applied to the two boxes separately. For each box, the relevant pieces of the environment are the rope (tension) and the earth (gravity). If the pulley is massive, it may be treated as a third system with the [tension] from the ropes as an interaction (in this case represented as torques).
System Specification
In solving a mechanics problem, the system is specified by stating which objects will constitute the system, whether those objects will be treated as point particles, rigid bodies, or massless objects. Note that the environment is not part of the system. The environment is described through its interactions.
Some problems (like the Atwood's Machine described above) may require more than one system to be considered.