what is the difference between a cosigner and a co borrower?

None, really.

A co-signer is someone who guarantees the loan but has no other interest in the property/money.

A co-borrower is someone who has an equal responsibility for the loan.

A co-signer is someone (usually a parent) who basically signs something which forces them to make your repayments on a loan or similar if you can't or won't. Generally people with no or bad credit are required to get a co-signer if the lender has any doubts about their ability to make the payments. A landlord might also require a potential tenant to have a co-signer.

A co-borrower is when another person is named on a tenancy agreement, loan or mortgage with you. So a spouse or friend. Both incomes are taken into account, which isn't the case with a co-signer.

Cosigner means that if the person can’t make the payments then you’ll step in and make the payments.

Co-borrower means both of you are borrowing money and both of you will make the payments.