when you hear in the papers or on the news something being described as a "think tank" they are typically referring to nothing more impressive than lobbying organisations, these are either self funded or funded by the organisations they seek to support. They typically just aim to push a certain policy and try to encourage / force policy change within government, they will conduct research but as they are based around certain set policy ideas the research can not often be trusted as it will almost certainly be bias.
They like to be called think tanks as it makes them seem official / recognised
i always think of it as another name for 'committee'.
nobody likes to hear about committee meetings any more, they sound beaureaucratic and dull, and full of people only atending so that they can get free tea, biscuits and expenses. think tank is the modern equivalent. thats what i think anyway.
It depends on the Think Tank, really. But on average they're just research organisations of one kind or another - some of them are very, very useful. They all have their own goals etc. but they're a damn sight more reliable in their chosen fields than most of the press in this country. You just have to be a little careful.