What on earth is an activist investor?

From the Wall Street Journal:

One of America’s oldest corporate law firms is diving into the business of representing activist investors, betting that these agitators are going mainstream—and offer a lucrative business opportunity for advisers. Cadwalader, Wickersham & Taft LLP has hired a new partner, Richard Brand…

Whoah, hang on a second there…what’s an activist investor?

Here’s a short video, telling you everything you need to know…