Motion of individual charges in magnetic fields
The force exerted on a charge is always perpendicular to the velocity of the charge.
The force exerted changes the velocity so the direction of the applied force changes as a result.
Overall this results in a circular motion. For example, if an electron moving left to right across a screen enters a magnetic field that has field lines into the screen, then the initial force on the electron will be downwards. But as the direction of the electron's movement changes, the force will keep changing. So it provides a force in a radial direction towards the centre of a circle.
This can be demonstrated with an electron deflection tube.
This is why many of the tracks of particles in collider experiments are circular 'spirals'.
This is also the reason that the large hadron collider at CERN in Switzerland is circular, as seen from above:
