What is populism anyway?

Not a day goes by without the word being read, heard, uttered in the media or our discussions: PO-PU-LISM, that political trend to which Donald Trump is said to have given his most resounding victory. Elsewhere, Podemos, Viktor Orban, Beppe Grillo, Marine Le Pen or Christoph Blocher would be its other faces. But what kind of reality does this term refer to? Is populism really taking over our democracies?