Recto and verso are used to indicate the "front" and "back" of a leaf of paper in a book. In Western books that are read from left to right, the recto is the page on the right hand side, the "front" of a leaf. In the same open book, the verso would be the page on the left hand side, i.e. the "back" of the facing page. If you were to turn the right-hand recto page in a normal fashion, you would be revealing a new verso and a new recto—that is, the back "verso" of the leaf you just turned, and the front "recto" of a new leaf.

See also: sheet > leaf/leaves > folio/foliation > pages/pagination