Single processor Quad core

This kind of servers will give slight enhancement than a desktop PC. Will feature ECC RAM and better cabinet and its more reliable than a desktop. Normally Used for connecting  5 -10 Client PCs.

Single processor Xeon

This will have server processor like Xeon and will have dual network card, basic server management, more memory slots and possibility of both SATA and SAS interfaces. You can have RAID features. Ideally suited   to 5-25 clients with simple application.

Dual processor Xeon

This platform gives all the features of a professional server. Dual processor, SAS HDDs ,more memory, server management features , suited to 25-100 clients   or users.

4-way server

These are Departmental servers and can be used as  high end server  used for Database applications,ERP etc. This can populate up to 4 processors and each processor can be of six cores.

N-way server

Some brands like IBM has got N-way server feature where you can connect multiple server to act as a single server. High performance online database applications will find great advantage from this.

RISC servers

IBM and SUN has got RISC architecture server. Financial institutions and high end applications on Unix/Linux platforms opt for this platform.

Tower server and Rack server

All the above servers come in tower and rack form. The advantage with the rack is that you can manage multiple server switch KVM switch with a single keyboard and mouse.

Blade server

Advantages : if you require 3 or more servers this gives benefit in terms of cost, power consumption, compact and load balancing features. Also you can add a storage module to the enclosure for increased storage requirements.