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.
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.
Some brands 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.
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.
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.