Saul Eslake at Club Troppo:
"One day last week I came into the office to find an email from my boss time-stamped 2:46am (and no, he wasn’t in another time-zone) asking ‘what, technically, is a depression’. What follows is a slightly expanded version of my answer.
There is a very old joke which says “a recession is when your neighbour loses his job, a depression is when you lose yours” . This plays to the widely accepted contemporary belief that a depression is simply a particularly severe recession. A quick trawl through cyberspace (which can be done readily by googling ‘difference between recession and depression’) throws up two criteria for distinguishing a ‘depression’ from a ‘recession’ – a ‘depression’ is either a decline in real GDP of more than 10%, or a contraction in real GDP which lasts more than three, or four, years."
The full thing's here.
And here is the Great Depression explained. The entire encylopedia is good too.
And Wikipedia's account of The Global financial crisis of 2008