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A directory resembles a dictionary or a phone book. If you know a word, you can look it up its entry in the dictionary to learn its definition or its pronunciation. If you know a name, you can look it up its entry in the phone book to find the telephone number and street address associated with the name. If you are bored, curious, or have lots of time, you can also read through the dictionary, phone book, or directory, entry after entry.
Where a directory differs from a paper dictionary or phone book is in how entries are indexed. Dictionaries typically have one index: words in alphabetical order. Phone books, too: names in alphabetical order. Directories entries on the other hand are often indexed for multiple attributes, names, user identifiers, email addresses, telephone numbers. This means you can look up a directory entry by the name of the user the entry belongs to, but also by her user identifier, her email address, or her telephone number, for example.