The simplest form of servers are disk servers and file 
servers. With a file server, the client passes requests for 
files or file records over a network to the file server. 
This form of data service requires large bandwidth and can 
slow a network with many users down considerably. 
Traditional LAN computing allows users to share resources, 
such as data files and peripheral devices, by moving them 
from standalone PCUs onto a Networked File Server (NFS).

The more advanced form of servers are database servers, 
transaction server and application servers (Orfali and 
Harkey 1992). In database servers, clients passes SQL 
(Structured Query Language) requests as messages to the 
server and the results of the query are returned over the 
network. The code that processes the SQL request and the 
data resides on the server allowing it to use its own 
processing power to find the requested data, rather than 
pass all the records back to a client and let it find its 
own data as was the case for the file server. In 
transaction servers, clients invoke remote procedures that 
reside on servers which also contains an SQL database 
engine. There are procedural statements on the server to 
execute a group of SQL statements (transactions) which 
either all succeed or fail as a unit. The applications 
based on transaction servers are called On-line Transaction 
Processing (OLTP) and tend to be mission-critical 
applications which require 1-3 second response time, 100% 
of the time and require tight controls over the security 
and integrity of the database.  
The communication overhead in this approach is kept to a 
minimum as the  exchange typically consists of a single 
request/reply (as opposed to multiple SQL statements in 
database servers). Application servers are not necessarily 
database centered but are used to server user needs, such 
as. download capabilities from Dow Jones or regulating a  
electronic mail process. Basing resources on a server 
allows users to share data, while security and management 
services, which are also based in the server, ensure data 
integrity and security. 

1.File server
2.Database Serevr
3.Groupware server
4.Object server
5.Transaction server
6.Web server
7.Appilication server

Servers are computer that provides the services. As for 
Example - 
1. DNS Server
2. WINS Server
3. DHCP Server
4. RAS Server
5. VPN Server 
6. File Server
7. IIS server
etc