Keynesian Economics

Need a little help here.

Could someone explain to me what this is all about?
- Advantages
- Disadvantages
- Which countries use them and why?

I've read some stuff about it online but I don't understand some of the terms that are being used. Thanks.

That is an oversimplification of Keynes's ideas.  Governments have since the 1930s done only one half of the things Keynes recommended.

Spend fiat money to stimulate the economy in times of recession and depression:  yes

Put surplus money in a national emergency fund during good times so that the nation doesn't have to go into debt to do that in the next recession:  no