Question

What is the difference between a linear velocity from a tangential velocity and radial velocity?

Answer 1

Linear velocity is the "real" velocity.

Explanation:

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).

In short:
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.