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.