Germans tend to be more formal when it comes to inviting people over. Unannounced or casual visits are not common.
If you have been invited by Germans, you can expect that the visit has been carefully prepared. The house will have been cleaned and tidied up. Host and hostess will be dressed nicely. If you are invited in the afternoon, at least one cake will be served with coffee (“Kaffee und Kuchen”). If it is dinner time, the dinner will be ready or almost ready when you get there. Do not arrive more than ten minutes late. Germans are very punctual and it is regarded as rude to arrive late.
In the afternoon, it is customary to bring a bouquet of flowers or chocolates, or both. Remember to take off the wrapping before presenting the flowers to the hostess. In the evening you can bring a bottle of wine which can be purchased in a regular grocery store, or once again, flowers or sweets.
Wait for the hostess to sit down at the table with everybody or to urge you to begin eating before you begin to eat dinner. Children are usually expected to also sit down at the same table to eat dinner and are not permitted to leave until they get permission from their parents to get up.
To find out how the table manners are different, please read my post about “Eating Out.”