A signature is a group of pages that are printed, most likely on both sides of a single sheet of paper that once folded, trimmed, bound and cut, become a specific number of pages. If you are printing an 8-page booklet, then you would have an 8-page signature. Signatures are always divisible by 4. You cannot have a 14- page booklet unless you have fold-out pages that are not bound into the spine. Even if you do specify a fold-out page, the number of pages will always be an even number (divisible by 2) because a sheet of paper always has two sides.
Closely tied with a print signature is a less common term that is known as imposition.This is the placement and direction of pages that are in a signature. Sometimes pages on a press sheet will seemingly be upside down or page 2 won’t be next to page 3, but when folded the pages will be in their proper position and sequence. All the margins will be in alignment. It is the job of the Print Estimator to set up a signature’s imposition.
Printers will often speak of two kinds of spreads: reader spreads and printer spreads. A reader spread is what the reader sees in the finished product, like when page 112 is opposite from page 113. Most designers design in reader spreads so they can see exactly what the end product will look like. The pre-press department then has the job of switching the designer files into printer spreads. Printer spreads are where those pages are when they are printed on a flat sheet of paper. It is possible that pages 112 and 113 might be next to each other, but it is also entirely likely that they could even be on different press sheets.
The important thing to remember is that when it is bound together, when a job is laid out correctly, the right pages will almost magically turn up next to each other. The more the number of pages in a job, the tougher it is to lay it out correctly. A proof is always sent out before any printing is actually done and one of the things to check for is that all the pages have ended up in the correct order.