G'day, I'm T1, but Basil refers to a long term background insulin lasting approximately 1 day, where-as Bolus it typically a mealtime dose of faster acting insulin.
Ahhhh probably obvious but you take it via an injection using a syringe or insulin pen.
Good for? Basil insulin smoothes out out your blood sugar levels with much less risk of hypoglycemia. Think gradual change as opposed to rapid.
Many take their Basil insulin at night as I did for 25 odd years but I've switched to mornings with better results. I can't remember the reasons for the change possibly because of inconsistent bedtimes ( e.g. out partying ;-) ).
NB: I am not a Doctor and anything I say should not be taken as medical advice .... it's just my 2p worth.
Summary:
Basil = long acting insulin
Bolus = fast acting insulin