The physics
A force turns a body about a pin. Its turning effect is the torque τ.
τ = F r sin θ = F r⊥
r⊥ is the lever arm. It is the distance from the pin to the force's line. A force along a line through the pin gives no torque.
Torque is a vector. Curl your right hand's fingers the way the force turns the body. Your thumb points along τ.
τ = r × F v = ω × r
A spin is a vector too. ω points along the axle, by the same right-hand rule.
A body moving past a pin has angular momentum about it. For a small body, L = m v r⊥.
L = m (r × v) τ = dL/dt
A torque about a point changes L about that point. With no torque, L stays the same.