A co-signer and a co-borrower have a lot of similarities. Both guarantee that the borrower will pay off a mortgage loan. If the borrower does not make payments, the co-signer or co-borrower will be responsible for making the payments.
But the one major difference is that the co-borrower is on the title of the property whereas the co-signer just has responsibility for the loan.
From the perspective of a lender, there isn't much difference between a co-signer and a co-borrower, because both guarantee the loan payments.
Co-signers are usually relatives whose financial profile can help a borrower get a mortgage loan. The lender will consider the co-signer's income and debt in making the loan, as well as the borrower's income and debt.
A co-borrower might be a business partner or a relative, whose financial profile helps to secure the loan, but the co-borrower has a share in ownership of the property and a name on the title. Co-signers can appear on lots of different types of loans including personal loans and car loans.
People should only become a co-signer for those well-known to them and people known to be responsible. That's because the consequences of a loan default fall heavily on the shoulders of a co-signer. In the case of a mortgage, the co-signer is responsible for the whole mortgage if the borrower defaults. Even if the co-signer pays off the entire mortgage, the cosigner will not own the house — the borrower will own it. The co-signer will not even be able to sell the house to recover the cost of the mortgage, although the cosigner might well be able to take some legal actions to recover money.
On the other hand, a co-borrower (whose name is on the title) might be able to sell the property to pay off a mortgage, if all owners, including the borrower in default, agree.
In either case, co-signing and co-borrowing are usually done by people with close ties to one another.