Literally, "constitution protection". Institutions subordinate to the federal and state government(s) of Germany whose job it is to protect the country from any internal threats against its freiheitlich-demokratische Grundordnung, "free and democratic basic order".

However, the organization has no enforcement authority and is only allowed to observe persons and organizations that are considered a possible threat to aforementioned basic order. It can do things like phone tapping (needs special authorization) und use undercover agents, but cannot arrest anybody or search their home. But most information (about 80%) is simply gathered from public sources, like newspapers or magazines published by observed organizations, or public gatherings held by them.

If the Verfassungsschutz acquires information that requires further action (e.g. about planned crimes), it immediately informs police, public prosecution or government. Additionally, it publishes a yearly general report about its observations, which is available to anyone (can be bought in bookstores or downloaded).

The activities and organizations observed by the Verfassungsschutz are:

Additionally, the Verfassungsschutz uses its expertise in fighting organized crime and counter-espionage.

In general, having an organization like this is a good idea, and the lack of enforcement authority prevents it from becoming a threat to freedom itself, like the FBI under J. Edgar Hoover. Of course, it looks different if you're at its business end and always have this van parking outside your house andyour trash sifted through because you think that communism is a good idea...