The two most common and practical chimney caps are single flue chimney cap and multi flue chimney cap. You can see both of these in the pictures below.
Chimney caps protect your chimney from weather, leaves and animals. Rain, snow, and ice will enter into the chimney flue if no cap is installed. Worst case scenario is that water will be leaking into the home, whether in the basement or soaking through the masonry walls, causing damage to the interior ceilings and walls.
Leaves and twigs from trees is another cause for concern. Over time, a build up of leaves will block the bottom of the chimney, causing the dangerous flue gasses to back up into the home.
Many times in the off season, when the chimney is not being used, animals like to crawl in the chimney. Most times they will get stuck and die inside your home. These could be birds, squirrels, raccoons and bats, just to mention a few. It is very common for them to build nests, causing a blocked chimney.
The purpose of a Multi-Flue chimney cap is to cover multiple flues at one time and protect the crown of the chimney. This is not only more ascetically pleasing but is functionally more effective. This give more protection to the chimney than a single flue cap could. The cap can be built to your specific chimney size. This way it will ensure a proper fit, giving the best possible look and protection at the same time.
All of the problems can easily be avoided with a high quality stainless steel chimney cap.