Directories are alphabetical lists of names and addresses. Some German directories list all the adult residents or tradesmen who lived in a town or area at the time the directory was published. The earliest German directory, for the city of Halle, was published in 1701. Directories have become common since then.
The most helpful directories for genealogical research are town directories of local residents and businesses.
These have usually been published annually and may include names, addresses, occupations, and other helpful facts. Particularly in large cities with several parishes, addresses often help you find your ancestor's parish. Directories sometimes have town maps and may include addresses of churches, cemeteries, civil registration offices, and other locations of value to the genealogist.
Modern telephone directories with over 32 million addresses and telephone numbers are now available online at:
A list of many German city directories is found on pages 235 to 261 of Ribbe's Taschenbuch für Familiengeschichtsforschung.
There are city directories online at http://adressbuecher.genealogy.net/book/list
The Family History Library has a few German city directories. Most date from the 1830s to the early1900s. They are listed in the Place Search of the catalog under:
GERMANY, [STATE] - DIRECTORIES
GERMANY, [STATE], [TOWN] - DIRECTORIES