For a Standard Quinella, choose the first two placegetters past the post, in any order. As long as your two selections finish first and second, you collect the Quinella dividend.
With a Box Quinella you can choose as many runners as you like, providing two of your selections finish first and second, in any order. The more runners you 'Box' in a Quinella the more combinations you get.
If you feel there's a 'sure thing' in a race that looks certain to win or at worst run second, then a Banker Quinella is a perfect type of bet. Your 'Banker' would be the 'sure thing' and you can group it with any number of other runners in that race. Providing your 'Banker' runs first or second and any one of your other selections finishes first or second with your 'Banker', you win the Quinella. The more 'other' selections you take, the more combinations you get.
A Multiple Quinella allows you take as many runners as you like for each placing. For example, you can take three (3) runners for first or second, and five (5) runners to fill the other placing. Provided that one of your runners selected in the first leg places either first or second, and one of your runners selected for the second leg fills the other placing (second or first), then you win the Quinella.