A HELOC, or home equity line of credit, is a revolving line of credit with a variable interest rate and, as a result, variable minimum payment amounts. With a HELOC, you can withdraw money as needed, up to a certain limit, and pay the interest only on the amount you borrow.
A second mortgage is a separate loan taken out in addition to your primary mortgage. The loan comes with fixed payments and a fixed interest rate for the term of the loan. The funds are disbursed in a lump sum, and you must start making monthly installment payments on the loan immediately until the loan is paid off.