The generic example of a directory would be a telephone directory or an address book. We use white pages directories when we need to find something specific about a person or a business about which we know something distinguishing such as the name of the person or the name of the business. When we need to find more general information about a group, say we need to find the list of all local merchants who specialize in selling reusable widgets, we refer to a yellow pages directory.
We use directories when we use e-mail or a web browser. The e-mail client sends a mail message to a mail server. The mail server looks up an internal table to locate the host machine on which the recipient of the message has an account. Similarly, when the name of a website is typed into a browser, the browser contacts a Domain Name System (DNS) server. The DNS server looks up an internal table that maintains a mapping between a DNS name and the IP address of the machine that hosts the web site. The server returns the mapping to the browser that now talks directly to the web server using its IP address. Ideally, such information can reside in a directory and can be accessed by any client that can speak the protocol of the directory. In fact, there are now quite a few installations where DNS lookup is directory enabled using DNS-to-LDAP gateways.