What is the difference between a linear velocity from a tangential velocity and radial velocity?
Linear velocity is the "real" velocity.
Since we cannot measure the linear velocity of a far-away object, we split it up in the radial velocity, which we can measure by the Doppler-effect (so-called red shift), and the tangential velocity, which we may measure by parallax (shift against the star background).
Radial velocity is the speed towards or away from us.
Tangential velocity is the velocity across our field of view.
With these a vector-rectangle can be set up, with the linear velocity being the diagonal.