Paper shredderpaper documents by cutting the paper into very fine strips or tiny paper chips. Many shredders simply cut documents into fine strips of paper, though cross-cutting shredders will cut the strips horizontally as well, resulting in small rectangles which are more difficult to piece back together.
It's important not to feed shredders more paper than they can handle; otherwise the motor may burn out and the shredder quit working. It's equally important to avoid feeding them items of clothing (such as neckties), as it will dull the blades and generally be uncomfortable for any humans involved.
For maximum security, documents should be shredded so that the words of the document go through the shredder horizontally. Many of the documents in the Enron accountancy scandal were fed through the shredder the wrong way, making them easier to reassemble.