This is the conversion that turns harsh-tasting malic acid into softer lactic acid.
Lactic bacteria are responsible for the conversion. The bacteria like to operate in a warm cellar and traditionally this meant winemakers had to wait until the spring following the harvset for the malolactic fermentation to occur. However, modern cellars can be heated up to allow the malolactic fermentation to start as soon as possible. In addition, many commercial wines will have lactic bacteria (from a packet) added to the wine as soon as alcoholic fermentation is finished.
Some wine styles are not put through malolactic fermentation as producers want to keep the zingy acidity in varieties such as Riesling and Sauvignon Blanc. To stop the malolactic occurring, winemakers will add sulphur and cool the wines.
Wines high in acid/low pH, which would benefit from malolactic fermentation are ironically the most unliokely to go through malolactic as the bacteria does not like high acid environments.
All red wines go through the malolactic fermentation