The physics
A magnetic field pushes only on a moving charge. The push is at right angles to both the velocity and the field.
F = q v × B F = q v B sin θ
Point your fingers along v and curl them toward B. Your thumb gives the push on a + charge. An electron's charge is negative, so its push points the other way.
The push never speeds a charge up or slows it down. It only turns it. Across a uniform field, the path is a circle.
r = m v / (q B)
A field is electric or magnetic depending on how you move. A rider moving with the charge sees an electric field E′ = v × B instead.