These days, none. There used to be a definition that ale was beer made without hops, but that's been long out of use. The only differences nowadays are between beers and lagers, which have different fermenting processes. Ale/beer uses a 'top fermenting' method where the yeast works at the top of a relatively warm tank. Lagers use a 'bottom fermenting', where the yeast works at the bottom of a much cooler tank. This lower temperature means that lager is slower to produce, and that's why it's more expensive.