Well, it is the party that takes place all over Germany before Lent starts. It is the German equivilant to Mardi Gras, or Carnival. The main event is the street carnival that takes place in the period between the Thursday before Ash Wednesday and Ash Wednesday. Carnival Thursday is called "Altweiber" (old women) in Düsseldorf or "Wieverfastelovend" (The women's day) in Cologne, in many places Carnival-"fools" take over city halls or municipal government, "wild" women cut men's ties wherever they get hold of them. On the following days, there are parades in the street organized by the local carnival clubs. The highlight of the carnival period however is Rose Monday (Rosenmontag). Although Rose Monday is not an official holiday in the Rhineland, in practice most public life comes to a rest and almost all workplaces are closed and shops are open only in the morning or not at all. The biggest parades are on Rose Monday, the famous "Rosenmontagszug" (Rose Monday Parade), e.g. in Cologne, Düsseldorf and many other cities. During these events, hundreds of thousands of people celebrate in the streets at low temperatures, most of them dressed up in costumes. Almost every town has a special carnival cry (Cologne, Bonn and Aachen: Alaaf!; Frankfurt, Düsseldorf and Mainz: Helau!; Mönchengladbach: Halt Pohl! (hold on to the pole); Rheydt: All Rheydt!).
If you are going to be in any of these cities during this time, bring a costume, be prepared to see lots of drinking and the behaviour that goes with it :-)) and a bag to collect all the candy and what not they throw from the floats during the parades. Parades are on different days in different cities, so check first. I know Frankfurt is on Sunday, and Mainz and Köln are on the Monday. Almost every city has a parade of some sort and they can be a lot of fun.