"It is my bag."
Translation:Es mi bolsa.
Yes, both words exist.
Bolso is normally referred to a hand bag, generally those carried by women, but also men bags and the like.
Bolsa applies to those flimsy bags made out of a unique layer of polyethylene molecules that will tear your hands in 10 mins, garbage bags and mainly plastic and grocery bags.
When referring to holdalls, gym bags, laptop bags, both are almost interchangeable, but they are identified (la bolsa del gimnasio,el bolso del ordenador, ...)
Backpack is Mochila though, although it might also be referred as bolsa o bolso sometimes
Beware there might be localisms out there... whilst this is true, it might not be the whole ;)
Holdalls. A term I never heard before. It would be a great addition to American English.
Is this true in Spain? Because at the supermarcato here im pretty sure i have only heard it called bolso.
I know that it is literally "the bag is mine", rather than "it's my bag" but I have heard it used so often I thought it would be accepted given the meaning is the same.