Introduction
LDAP is not a new technlology. It has been around since mid 1990, as a way to mitigate the complexity of the X.500 based servers access. It's name is an acronym for Lightweight Directory Access Protocol. Soon after the first standard has been issued, the first full LDAP server was written (ie, X.500 was pushed out of the equation).
We will now use the term LDAP and LDAP server for respectively the protocole and the server.
Features
A LDAP server provides access to entries, stored in a backend. It offers an interrogation mechanism allowing fast retrieval of entries. The data structure is hierarchical, and we use a schema to manage the content of entries, plus the organisation of data.
A LDAP client first has to connect to the server, and disconnect at the end. Some operations can be done on data, searches, modification and deletion, among a few others.
LDAP servers are extensible, but they all use a common protocol which makes it easy for users to request them. This API is an exemple of what LDAP is very good at : access data in a fast way, across servers.
Characteristics
LDAP servers are fast for retrievals : they have been designed for this purpose. On the other hand, modifications can be costly. This has to be understood when writing an application using a LDAP server as a backend.
Each entry is identified by it's position in the hierarchy, and we use what is called a Distinguished Name (or Dn) to describe this position in the tree. The base is also named the DIT, or Directory Information Tree.
Programming
Nowadays, LDAP is a part of the IT and it's difficult to avoid having to deal with it. LDAP servers are used to manage authentication, mainly, but also authorization, and more. It's very likely that you will have to write some code to access such a LDAP server, and the existing API are a bit cumbersome. This new LDAP API has been defined to facilitate this kind of tasks.
Going further
This was a very short introduction, you can find more literature about LDAP on the web : Wikipedia gives you a good starting point with many valid pointers.