An Activist Investor is an investor who purchases a not-insignificant stake in a company in order to pressure the board into making a decision that will make that investor a lot of money, generally in a very short period of time. The high-profile investors tend to want to shutter the company completely or to downsize immensely. Basically gut, maximize profits for a quarter or two, and sell.
Biggest example is probably Carl Icahn. He's well known in the tech world for trying to do these types of things to Apple, Netflix, and Dell. Not sure about the others, but Netflix actually has a Carl Icahn rule that prevents any one investor from gaining too much control. If I remember correctly, Icahn sold his stake almost immediately after the rule was put in place.
Netflix adopted a poison pill (http://money.cnn.com/2012/11/05/technology/netflix-poison-pi... - more professionally labeled a shareholder rights plan) to prevent Icahn from getting too crazy; he was basically threatening to make Netflix acquisition-bait. Icahn recently sold off his Netflix stake, but managed to make a ridiculous amount of money as the company saw some solid growth while he held a position.
Biggest example is probably Carl Icahn. He's well known in the tech world for trying to do these types of things to Apple, Netflix, and Dell. Not sure about the others, but Netflix actually has a Carl Icahn rule that prevents any one investor from gaining too much control. If I remember correctly, Icahn sold his stake almost immediately after the rule was put in place.