Effective Altruism (EA) is a movement trying to invest time and money in causes that do the most good per some unit of effort. The label applies broadly, including a philosophy, a community, a set of organisations and set of behaviours. Likewise it also sometimes means how to donate effectively to charities, choose one's career, do the most good per $, do good in general or ensure the most good happens. All of these different framings have slightly different implications.
The basic concept behind EA is that one would really struggle to donate 100 times more money or time to charity than you currently do but, spending a little time researching who to donate to could have an impact on roughly this order of magnitude. The same argument works for doing good with your career or volunteer hours.
The Effective Altruism movement also has its own forum, The EA Forum. It runs on the same software as LessWrong....