Many knives feature serrated blades. The saw-like cutting action of these knives allows for aggressive slicing and the edges hold up well. Eventually though every edge becomes dull and loses its cutting ability. Sharpening a serrated knife is much different than sharpening a traditional straight blade. Each individual serration will need to be sharpened separately for maximum effectiveness.
How to Sharpen a Serrated Knife
Using the DMT Serrated Knife Sharpener