Sour beer is a beer that is made with spontaneous fermentation. Wilde Gisten take care of this. They are often aged in barrels and have a very distinctive sour, bitter and sometimes even savory taste.
Geuze beer is 'paired' with old and new lambic. This Belgian beer type is spontaneously fermented and therefore one of a kind. The taste is fresh and sour and not very bitter. A commercial gueuze may be sweet, but a real old gueuze is not.