As we said before, most databases that
you will come across these days will be relational databases.
However, there are many types of relational databases and not
all of them will be useful for web applications.
In particular, it will be the client/server
databases rather than the stand-alone packages that you will use
for the web.
A client server database works like this:
A database server is left running 24 hours a day, and 7 days
a week. Thus, the server can handle database requests
at any hour. Database requests come in from "clients" who
access the database through its command line interface or by connecting
to a database socket. Requests are handled as they come in and
multiple requests can be handled at one time.
For web applications which must be available
for world wide time zone usage, it is essential to build upon a
client-server database which can run all the time.
For the most part, these are the only types
of databases that Internet Service Providers will even provide.
However if you are serving web pages yourself, you should consider
many of the excellent freeware, shareware or commercial products
around. I myself like postgres for UNIX since I prefer a
UNIX-based web server. However, there are plenty of good
applications for PC and Mac as well.