What are activist investors, and are they a good thing?

I seem to see the word "activist" more and more often in the business pages of the newspaper. There was a time when the word only cropped up in the news and environmental sections, but nowadays "activist investor" and "shareholder activist" are commonly-encountered phrases, suggesting that the practice is on the rise. But what does it actually mean?
An activist investor is a wealthy individual or a company or group that buys up significant chunks of a public corporation's shares, with a view to effecting major change in the company and it's management. This may also be accompanied by an attempt to obtain seats on the company's board of directors. Such a move may be precipitated by a perceived mismanagement, or excessive costs, or some other reason why the company is considered to be undervalued, although it can also be used to bring about social change, often related to the environment, investment in politically sensitive parts of the world, and workers' rights. Among the best known individual activist investors are Carl Icahn, Nelson Pelts, Bill Ackman, David Einhorn and Dan Loeb.
As for whether they are a "good thing", that largely depends on your point of view. Activists may bring fresh ideas to the table, and they may be able to "hold management's feet to the fire", resulting in improved performance and higher stock valuations. Just the simple attentions of an influential activist investor may be enough to increase a company's stock price and valuation. For example, there is a phenomenon known as the "Icahn Lift", which refers to the rise in stock price that often occurs when prominent activist investor Carl Icahn decides to get involved in a company.
However, activist investors are not always right, and they are usually in it for their own profit and not necessarily for the public good. And, when they come to offload their investment, which they may do at the drop of a hat depending on the success of their intervention or their own investment horizon, there may then be significant downward pressure on the company's stock price.
So, with shareholder activism on the rise, the blessings are mixed. It may lead to better governance, better and more independent boards, socially-responsible divestment campaigns, or the enforcing of environmental standards. It may, though, be extremely disruptive, and not all activist campaigns are successful or even desirable. As always, caveat emptor applies.