Action cams are small, lightweight, wearable, mountable, portable, and sometimes waterproof camcorders. They're useful because you can mount them to pretty much anything—from skateboards, surfboards, bicycles, and drones, to helmets, body parts, and even your pets.
Sure, you can mount any camcorder that may feature better functionality and performance for the price, especially since camcorders and most action cameras share the same standard screw hole for mounting. But regular camcorders are too heavy and bulky to strap onto yourself, your apparel, or your equipment. Plus, the gap between traditional camcorder and action camera performance is narrowing as technology improves. Action cams are forever getting smaller, lighter, and less expensive.
A few other notes on choosing an action cam: Waterproofing is important to consider if you'll be recording footage either underwater or around water. Some cameras can go deeper than others, and some have built-in waterproofing, so that you don't need to think about extra housings. You'll also need to consider frames per second (fps). Some action cameras offer up to 60fps or even 120fps recording, while others only go up 30fps. For standard playback, 30fps is perfectly fine. It's when you want to slow footage down in editing to create dramatic scenes that fps really matters.
By Eugene Kim, http://au.pcmag.com/digital-camcorders-reviews-ratings-comparisons/26064/guide/the-best-action-cameras-of-2015