The positions work the same way as on a violin, although there are some fingering choices that will be different due to the difference between bowing and picking some phrases.
Position is shifted to reach notes higher than available in first position, and to make some fingering passages easier / different to play.
Shifting is accomplished by keeping the hand relaxed with no gripping of the neck and sliding up until the first finger is in the relative position. Generally you are aiming the first finger to the position even if you are playing a different fingered note.
Fingers have two frets each in the basic form, but this can change depending on the scale or phrasing of the music.
Here is a picture of C major scale in 1st 2nd and 3rd positions: