The purpose of voice leading is to create a smooth and fluid motion between chords.
Parallel Fifths
Parallel Octaves
Voice crossings
More than one octave between S and A or A and T (T and B is ok)
Melody
Consecutive skips in the same direction
Augmented intervals
large intervals
The Bass line is usually exempt from these restrictions
Voicing
Doubling the leading tone
Leading tone (that belongs to a V or vii) goes up
Good tendencies:
leave common tones in the same voice
avoid skips