In the X.500 directories, DAP is the protocol used to query the directory. LDAP, then, is just a lightweight version of DAP, which is only one part of the bigger directory service picture. All other aspects of building, managing and using a directory are left to the developer's imagination. One of the confusing things about LDAP is that it looks like another kind of database. If you've had experience with SQL or any relational database, LDAP looks like a way to get at a simple kind of database.
First impressions are correct.
You can use LDAP to retrieve data from a standard database, as Oracle and Computer Associates have demonstrated with their LDAP directories, which have been built on top of their relational databases. LDAP server vendors, such as Netscape and Innosoft, don't like this comparison. For one thing, they don't want to compete with established database vendors. Selling database software against Oracle is only slightly easier than selling operating system software against Microsoft. However, they also want to emphasize the differences between their directories and a traditional relational database.