A bass guitar in general have frets on the fretboard. The size and the number of frets may vary from model to model.
A fretless bass does not have any frets on the fretboard. One can remove the frets of a regular bass guitar or buy a fretless model.
A 4 string bass is the most common bass guitar. It's standard tuning being E-A-D-G.
A 5 string bass includes an extra string to the regular 4 string. It consists of a lower pitched thicker string - usually tuned to the note 'B'.
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