Basically, there are two categories of beer: ales and lagers. How do they differ? The secret is in the brewing process and not the appearance of the beer as you may have thought.
The first difference is that ales are top-fermenting which means the yeast ferments at the top of the container. These yeasts produce esters (a class of compounds produced by reaction between acids and alcohols with the elimination of water) which can affect the flavour of the beer.
Lagers on the other hand, use bottom-fermenting yeasts which sink to the bottom of the container. Collecting these yeasts from the bottom for reuse is fairly simple and adds to the “greeness” of the process.
The second difference is that ale yeasts fermet better at warmer temperatues, generally room temperature. This fermentation process tends to be faster than lagers since lagers ferment at colder temperatues (around 15C).
The final difference is in the ingredients and hence the taste. Ales tend to contain a higher amount of hops, malt and roasted malts giving them their malty, bitter taste.