Strictly speaking, a database is any collection of information that
can be organized according to some set of attributes. This isn't
limited to data stored on a computer, there are many examples of
databases that are not computerized: card catalogs (obsolete), filing
cabinets, wallets, newspaper stock listings, microfilm catalogs, almanacs,
encyclopedias, and dictionaries are all databases. Even an alphabetized or
date ordered bookshelf qualifies as a database, albeit not much of one. Common
usage now restricts the term to computerized collections of data, though,
so I'll focus on those for the remainder of this write-up.
First, while most people mean a relational database when they use the
term database this is far from the only type of database that exists. There
are also flat, network, hierarchical, and object databases. There are also
specializations of the typical relational system such as temporal databases, data
warehouses (really just huge relational systems), and several types of
distributed relational systems that differ primarily in implementation
details.
Flat
A flat database is one where the entire database is stored
in a single file either on disk or in memory. It can
be something as simple as a plain text file (such as /etc/shells
on a linux box) or a binary data file such as a Berkeley DB. It
can be formatted any way that the user needs it to be, but all information
required for navigating the database is contained within a single
file. Flat databases include the aforementioned /etc/shells as well as
most address books and bookmark files.
Hierarchical
A hierarchical database is one where the data is laid out in a in a tree
fashion descending from a root node. One good example is a file system
with its hierarchical directory structure. The Java class hierarchy
can also be thought of as a hierarchical database.
Network
A network database is, surprisingly, a database where the information
is stored scattered about 2 or more nodes on a network. The foremost
example of a non-relational network database is the Domain Name System,
but any system where knowledge is distributed in a similar fashion is
a network database.
Object
An object database differs primarily by creating a notion of identity for
the data. Each record in the system has a unique identifier that tags
it. Everything else is supposed to use this identifier to refer to the
object, rather than using qualifications on the data the object contains.
Relational
Currently the type of database that most people mean when they say
database. A relational database system is just what it sounds like--a way
to organize data according to the relations between various pieces of
it. In a relational database the data is described as a series of tables
that each contain a set of fields. For instance, if you needed to keep
track of bibliographic data you might create tables called authors,
publishers, and articles. The articles table
would then have fields relating to both the publishers and authors
tables in addition to fields specifying things such as the article's length, subject
matter, or even the full text.
| 
