Technically a sour

Saisons/Farmhouse Ales are not sours, unless it's indicated on a label that it's a sour Saison. Gozu is not a style. Just Westbrook's Gose with yuzu fruit. So they called it Gozu. Other sours are Flanders Red Ale and Flanders Oud Bruin.

EDIT: Another sour style is Berliner Weisse

 

http://www.bjcp.org/docs/2008_stylebook.pdf
Chapter 17

 

It's sour if when you drink it, it tastes sour.

There are sour versions of so many styles, it's hard to make the determination based on style category alone.

 

Sometimes there are brett conditioned sours. some are tart or light sour character.

 

Prairie makes some good examples of "sour saisons". Prairie Ale, Prairie Puncheon, and Prairie Gold are slightly sour farmhouse ales. Prairie Somewhere is very sour.

 

The easiest way to answer OP's question without going down the rabbit hole of, "Any style can be sour," is to say, according to the style categorizations of this site, gose, Berliner weiss, lambic, fruited lambic, gueuze, Flanders red, Flanders oud bruin, faro, and American wilds.

There are also other styles that can be or tend to be sour but are not necessarily, such as saison, biere de garde, fruit beer, biere de champagne, and any of the "old world" styles like gruit, sahti, kvass, braggot, and herbed/spiced beers. Again, many will not be sour as it is only possible not necessary. OEC Brewing makes a point of souring some of their beers that would have been so in the past such as their bock and old ale. New Glarus also (in)famously made a classic, to-style old English porter that was sour because that's how the beer would've been found in its time.

Despite my earlier reticence, for the sake of completeness, really, it is true that any style can be sour if it is brewed with the requisite bacteria and/or Brett (yes, I'm aware that Brett apparently doesn't make sour, but let's let that pass for now). Wine barrel aging will also oftentimes sour a beer. I'm fond of sour stouts myself, and wine-barrel stouts are my favorite substyle. Prairie and Jester King both make some beers that are hoppy enough to be considered sour IPAs, in my opinion, but sour stouts/IPAs/etc. all fall under the catch-all American wild category.

 

"Sour" is a catch all term as vast as its defined:
1.having an acid taste, resembling that of vinegar, lemon juice, etc.; tart.
2.rendered acid or affected by fermentation; fermented.
3.producing the one of the four basic taste sensations that is not bitter, salt, or sweet.
4.characteristic of something fermented;a sour smell.
5.distasteful or disagreeable; unpleasant.

Totally agree with 95% of your post but the snippet that is bold and quoted above. "Brett" does indeed make something sour as a purest definition, as "Brett" produces a large amount of acetic acid during fermentation ( definition #2). Yes i know its nitpicking but if we are going to get "technical" as @BubalooBrewMaster requested.

 

If you are getting technical; within the Lambic category there are narrower definitions: Gueuze, Kriek, Framboise.

 

In general any beer to which intentionally lactobacillus or pediococcus have been added is a sour beer. These are the bacteria that actually sour beer.

 

Oo, don't forget about beers that get infected. Those can also be pretty sour, and occasionally surprisingly delicious.

 

Brett does not make sour beers. Someone mentioned it creating acetic acid, but that is typically considered a huge flaw if there's more than just a small amount, and this only happens in the presence of a decent amount of oxygen.

 

Any style can be sour, traditional styles that use souring acids by way of acid producing bacterial fermentations or acid-containing ingredients (ie. acidulated malt) are:

Berlinerweisse
Gose
Lambic
Geueze
Faro
Flanders Red
Flanders Oud Bruin

Sour = acidic. Acids lower pH of beer. Brettanomyces does not bring any significant amount of acid, just funk.

Clean beer pH 4.2 ish
Mild sour beer pH 3.6 - 3.8
Moderately sour beer pH 3.4 - 3.6
Strong sour beer pH 3.0 - 3.4
Extremely sour beer pH < 3.0

That's a quantitative scale I have referenced from my experience drinking and making sour beer.

 

The perception of sourness, much alike bitterness from hops, is affected by residual sugars in the beer and sweetness. A bone dry, fully attenuated beer that is pH 3.4 will taste much more sour than a semi-sweet beer that also has a pH of 3.4.

 

Here's a 2.5 min. video on sour beers providing a basic overview for initial insight into this genre of beer: http://www.monkeysee.com/play/28249-sour-beer

 

I think the answer you've been looking for has been answered, but it is important to know that "any beer style can be sour" is most definitely correct. As long as any bacterial strain that creates acidity has become active in the beer (lacto, sacchro, pedio, brett), the beer is now sour. And yes, even brettanomyces adds sourness, although it is very little to notice.

Most of what I'm talking about is commonly called an "infection," usually in reference to homebrewing, or infamously with the 2009 Abyss.

When it is done on purpose, it is the most enjoyable experience and proof to me that there is a God IMO