The traditional Biergarten is a Bavarian invention first developed in the mid-19th century around the city of Munich during the reign of Bavarian King Ludwig the First. Back then, the good people of Munich drank dark beer known in German as Dunkel Bier. These beers were cold-fermented, and before the invention of industrial cooling systems, brewing at a cool temperature was only possible during winter.
In order to serve cold beer during summer, Bavarian breweries began digging beer cellars into the moist banks of the river Isar, where winter time ice was used for cooling and stabilizing the temperature during summertime. To further secure stable cool temperatures inside the beer cellars, they planted chestnut trees (which provide lots of shade with their big and plentiful leaves) and laid white pebble stones on the ground to further reflect sunlight.
People would buy beer at these underground cellars to bring home in large pitchers with lids—to this day, original Munich beer mugs still have lids attached to them. It didn't take an Einstein to conceptualize the next step of putting up a few wooden benches and tables and serving the beer on top of the cellars. Surrounded by a beautiful garden of chestnut trees, listening to the flow of Isar while enjoying a few savory pork bites, it was a lovely place to be.
There are some important characteristics that define a Biergarten. The garden concept must be taken seriously. There has to be a garden/open forest feel, ideally achieved by sitting under a 150 year old chestnut tree overlooking a river or a small lake. There should be pebbles on the ground and long wooden benches and tables.
The Biergarten also serves other purposes beyond simply quenching thirst and hunger. Its importance as a social establishment promoting a feeling of solidarity, community and Gemeinschaft cannot be underestimated. In the rigid class-divided society of 19th century Bavaria, the Biergarten offered a rare opportunity for families from all walks of life to drink, eat, and mingle. To further advance this egalitarian idea Bavarian Law actually secured the right to bring and consume your own food while frequenting Biergartens thereby encouraging poor people to join.
Unlike bars, kneipen (pubs), and other watering holes, the traditional German Biergarten is a place for the whole family. Kids and grandparents alike are welcomed and there's even a playground or large sandbox inside. Over-the-top drunken behavior is a faux pas, and after many Biergarden seasons under my belt, I cannot recall ever seeing someone being obnoxiously drunk and escorted out.
Biergartens are found all over Germany. Some of the most beautiful I've visited outside Munich are located along the Rhine river banks near the Cologne/Bonn area overlooking rocky hillsides covered with wine stocks and medieval castles ruins. In Berlin, I recommend the Prater Biergarten, one of the oldest located at Kastanienallee in Prenzlauerberg, or the Cafe am Neuen See in Tiergarten, or the more touristy (but still nice) Schleusenkrug at the old water lock, also in the Tiergarten.
The typical Berliner Biergarten serves a wide range of beverages including wine, soft drinks such as Apfelschorle (apple juice mixed with sparkling water), coffee, tea, and of course, many different kinds of bottled beer and beer on tap. I'll usually get started with a few cold refreshing Kristallweizen (a straw colored and filtered version of a Hefeweizen) without the optional lemon, and a warm crunchy salty pretzel on the side. My better half will usually go for a Radler, which is a Pilsner mixed with Sprite.
A perhaps underrated feature of today's Biergarden is the extensive array of food available. In Munich you'll be served traditional heavy Bavarian dishes such as schweinshaxe with sauerkraut and potatoes, but some spots also serve huge pizzas alongside more traditional dishes such as Leberkäse with mustard and Berliner style potato salad.