German Mustard or "Senf" is made of different kinds of ground mustard seeds (mostly Sinapis hirta and Brassica nigra) mixed with vinegar, oil and various herbs or sweeteners.
German recipes also call for whole mustard seeds, which are mild and even nutty tasting before they are used in prepared mustards, heated or in marinades. White mustard seed is the mildest and used in Mustard Pickles ("Senfgurken") and in some sausages.
More on the history of mustard.
Styles of German Mustard
Bavarian Sweet Mustard - "Süßer Senf, bayerischer" or "Weißwurstsenf"
Made from coursely ground and roasted mustard seeds, with vinegar and honey, sugar or applesauce as a sweetener. The manufacturer Händlmaier leads the pack for sweet mustard, has a red top and can be found in US supermarkets.
Medium Spicy Mustard - "Mittlescharfer-" or "Delikatesssenf"
This is made with a yellow and brown mustard seeds and is the most popular style in Germany. Sometimes it is mixed with horseradish in the Eastern German states and Austria and called "Meerrettichsenf".
Sharp or Spicy Mustard - "Scharfer Senf"
This mustard style is the same as medium spicy mustard, but with a higher percentage of brown mustard seeds, which are hotter. Düsseldorfer Löwensenf is a very well-known manufacturer of German mustards.