Shell voicings are better known as 3-note chords. They are made up of the most essential notes (root, third and seventh) that define a chord (the fifth is omitted) .They are very useful for beginner guitarists who want to explore basic guitar comping rhythms.
They also help understanding harmony, how chords are built, what are the intervals that compose them. You will understand that shell voicings are a good introduction to more complex chords.
This lesson with tabs and audio provides the essential chord shapes and exercises for practicing guide tone chords (aka 3-note chords) on guitar.
This concept highly prized by solo guitar players consists in mixing chords and bass lines. It can be very useful for guitarists who want to accompany a singer or a soloist in a duo situation for example.
Guitar walking bass lines involve playing one note on each beat in order to make the link between the chords of a progression as a bass player would do.
They are usually played fingerstyle, basses are played with the thumb whereas the other chord tones are played with the fingers.
This free lesson with tabs and guitar shapes provides some easy examples.