Differences between servers

In English, a small definition of the word server (related to computing) is: a central computer from which other computers get information. Talking about computer architectures (related to software, networks, etc.) this term is often used, but it is often misunderstood. Confusion is also due to the fact that many current software let a kind of intersection of features that were attributed to a single server.
What is the official definition of computer server? Evidently there are several "official" definitions but today this one maybe is the complete one:

In computing, a server is any combination of hardware or software designed to provide services to clients. When used alone, the term typically refers to a computer which may be running a server operating system, but is commonly used to refer to any software or dedicated hardware capable of providing services.

And how many types of servers are there? In general, currently every computer need is defined and provided as service (in particular web service). Talking about computer architecture we tend to associate an XYZ Server to every XYZ service. Example: a software architecture in which we need to store all the data, so we need a database service, therefore it exists a database server. This links from Wikipedia list all server types

These two links to Wikipedia list almost all types of existing servers in computer science (linking relative descriptions):

http://en.wikipedia.org/wiki/Server
http://it.wikipedia.org/wiki/Server

Some particularly common software known as server are:

WebSphere: it refers to a set of IBM software, but in particular to WebSphere Application Server that is a platform for implementing interoperable web applications.

WebLogin: it is another Application Server made by BEA Systems under Oracle; like WebSphere from IBM, it is also surrounded by a wide family applications.

Apache Tomcat: there is a lot of confusion about this software. Basically, it is an open source environment. Basically, it is an open source environment developed for servlet, that is a servlet container. However, in order to run Java code you need a web server and because of this Apache Tomcat also includes Web Server features (that is an HTTP Web Server purely for Java). Therefore Apache Tomcat is a kind of pseudo-server defined web container and is currently one of the most used because of its simplicity for installation, configuration and usage.

JBoss: it is an open source Application Server based on Java Enteprise Edition (JEE) platform, it is developed by JBoss under Red Hat e it is supported by a wide web community. In the last versions it includes also Apache Tomcat 5.5 as web container.

Apache HTTP Server: it is the most used Web Server software. It is also know simply with Apache and it is developed by Apache Software Foundation together with a lot of other open source projects for the Web.

MySQL: one of the most widely used database system. Currently it is developed by Sun Microsystem. It is an RDBMS including Database Server features.

PostgreSQL: it is an open-source ORDBMS, it is widely used too. It includes Database Server features.