thread that is (marketed) as 'bobbin' thread is generally a thinner thread than what you may use for sewing- 60wt or higher (the higher the wt- the thinner the thread)
that way you get alot more thread on the bobbin and do not have to change bobbins as often- less starts/stops.
i have used bobbin thread for sewing before- it is a strong thread...nice and fine.