There is no single German word for “hello”. Which one you hear depends on the region and on how well people know each other, and a few of the local ones will mark you out as a visitor who has done their homework.
The standard greetings
Hallo is safe everywhere and is the easy default among friends. Guten Tag, Guten Morgen and Guten Abend are the standard, slightly more formal choices, and they fit shops, offices and strangers. When leaving, Tschüss is the everyday “bye”, and Auf Wiedersehen is the more formal goodbye.
How formal you should be often depends on whether you say du or Sie to the person, which is covered in Du or Sie?.
In the north: Moin
In the north, from Hamburg and Bremen up to the coast, you will hear Moin, and sometimes Moin moin. Although it sounds like “morning”, people use it at any time of day, and it works as a casual “hello” to friends and strangers alike.
Moin! Wie geht’s?
Hello! How are you?
In the south: Servus and Grüß Gott
Munich’s Marienplatz, Thomas Wolf, www.foto-tw.de / Wikimedia Commons (CC BY-SA 3.0)
In Bavaria and Austria, Servus is a friendly greeting that works for both “hello” and “goodbye”. Grüß Gott, literally “may God greet you”, is the polite greeting in Bavaria, Austria and parts of the south, and you will hear it in shops and from the older generation, whether or not the speaker is religious.
In Switzerland the greeting is Grüezi.
At work: Mahlzeit
Around lunchtime in some workplaces colleagues greet each other with Mahlzeit, short for gesegnete Mahlzeit, which wishes everyone a good meal. It is a greeting rather than an invitation to lunch.
Which one should you use?
Use Hallo or Guten Tag wherever you are, and enjoy the local word when you hear it. A Moin in Hamburg or a Servus in Munich from a visitor is usually welcomed, as long as it is said with a smile.