Although they're often mistaken for each other, bi-weekly payments and semi-monthly (or bi-monthly) payments are not the same.
Semi-monthly payments split your monthly payment due in two payments, typically paid on the 1st and the 15th. Bi-weekly payments, on the other hand, are due every other week.
That means that after a year of semi-monthly payments, you'll have made 24 payments (two for each month). After a year of bi-weekly payments, you'll have made 26 payments (one for every two weeks).
As a result, bi-weekly mortgage payments can help you to pay down your mortgage faster. That means you'll save significantly on interest charged over the life of the loan.
Here's a quick cost comparison for a $200,000 mortgage with a 5% interest rate over a 30-year term.
As you can see, the bi-weekly payment plan offers significant savings when compared to a semi-monthly or traditional monthly plan.