Typically accordian folds are simple zig-zag folds with 6-panels and two parallel folds that go in opposite directions. Each panel of the accordian fold is about the same size.
Variations include half-accordian folds where one panel is half the size of the other two and engineering folds where one panel is twice the size of the other two. Eight and 10-page accordian folds are also common.
Accordian folds are similar to the pleats on the musical instrument known as an accordion (note the different spelling).
Some tri-fold brochures, business letters, invoices, and monthly statements commonly use an accordian fold. This fold allows an address at the top or bottom of a typical portrait style letter or invoice to show through a window envelope, avoiding the need for address labels.