<h2><a name=\"two-tier\">What is two-tier architecture?</a></h2>
<p>The two-tier architecture is refers to client/ server architectures as  well, the term client/ server was first used in the 1980s in reference  to personal computers (PCs) on a network. The actual client/ server  model started gaining acceptance in the late 1980s, and later it was  adapted to World Wide Web <span>programming</span>.</p>
<p><img src=\"images/2-Tier1.jpg\" alt=\"2-Tier1.jpg\" /></p>
<p>According to the modern days use of two-tier architecture the user  interfaces (or with ASP.NET, all web pages) runs on the client and the  database is stored on the server. The actual application logic can run  on either the client or the server. So in this case the user interfaces  are directly access the database. Those can also be non-interface  processing engines, which provide solutions to other remote/ local  systems. In either case, today the two-tier model is not as reputed as  the three-tier model. The advantage of the two-tier design is its  simplicity, but the simplicity comes with the cost of scalability. The  newer three-tier architecture, which is more famous, introduces a middle  tier for the application logic.</p>
<h2><a name=\"three-tier\">What is three-tier architecture?</a></h2>
<p>The three tier software architecture (also known as three layer  architectures) emerged in the 1990s to overcome the limitations of the  two tier architecture. This architecture has aggressively customized and  adopted by modern day system designer to web systems.</p>
<p>Three-tier is a client-server architecture in which the user  interface, functional process logic, data storage and data access are  developed and maintained as independent modules, some time on separate  platforms. The term \"<em>three-tier</em>\" or \"<em>three-layer</em>\", as  well as the concept of multi-tier architectures (often refers to as  three-tier architecture), seems to have originated within Rational  Software.</p>
<p><img src=\"images/3-Tier3.jpg\" alt=\"3-Tier3.jpg\" /></p>
<p>The 3-Tier architecture has the following three tiers.</p>
<ol>
<li><strong>Presentation Tier or Web Server</strong>: User Interface, displaying/ accepting data/ input to/ from the user </li>
<li><strong>Application Logic/ Business Logic/ Transaction Tier or Application Server</strong>:  Data validation, acceptability check before being added to the database  and all other business/ application specific operations </li>
<li><strong>Data Tier or Database server</strong>: Simple reading and writing method to database or any other storage, connection, command, stored procedures etc </li>
</ol>