Your Mac will do lots of things right out of the box, but there are some things that it just can't do on its own - for those your Mac needs help. That help comes in the form of helper apps or "plug-ins".
Plug-ins add functionality that Apple doesn't directly support in the operating system or in some cases, they enhance the capabilities of existing functionality. A great example of the former is Adobe's Flash Plug-in. One of the best examples of a plug-in that enhances existing capabilities is another Adobe Plug-in, Adobe Reader. While your Mac can view PDF files using Preview, installing Adobe's Reader will allow Safari to open PDF's on the web seamless, rather forcing the operating system to handle it by launching Preview.
Have you tried to open a Windows Media video, on your Mac? WMV, (Windows Media Video), is one of the most popular formats for video on the web today, but your Mac can't work with them natively; it needs a plug-in. Below is a list of links to the top 5 plug-ins.