Explain the different tiers of 2 tier & 3 tier architecture?

Wikipedia explains it better then I could

From the article - Top is 1st Tier:

First, we must make a distinction between layers and tiers. Layers are the way to logically break code into components and tiers are the physical nodes to place the components on. This question explains it better: What's the difference between "Layers" and "Tiers"?

A two layer architecture is usually just a presentation layer and data store layer. These can be on 1 tier (1 machine) or 2 tiers (2 machines) to achieve better performance by distributing the work load.

A three layer architecture usually puts something between the presentation and data store layers such as a business logic layer or service layer. Again, you can put this into 1,2, or 3 tiers depending on how much money you have for hardware and how much load you expect.

Putting multiple machines in a tier will help with the robustness of the system by providing redundancy.

Below is a good example of a layered architecture:

_{(source: microsoft.com)}

A good reference for all of this can be found here on MSDN: http://msdn.microsoft.com/en-us/library/ms978678.aspx

Here is some help for 2Tier and 3Tier difference, please refer below.

ANSWER:
1. 2Tier is Client server architecture and 3Tier is Client, Server and Database architecture.
2. 3Tier has a Middle stage to communicate client to server, Where as in 2Tier client directly get communication to server.
3. 3Tier is like a MVC, But having difference in topologies
4. 3Tier is linear means in that request flow is Client>>>Middle Layer(SErver application) >>>Databse server and Response is reverse.
While in 2Tier it a Triangular View >>Controller>>Model
5. 3Tier is like Website while web browser is Client application(middle layer), and ASP/PHP language code is server application.

Tiers are nothing but the separation of concerns and in general the presentation layer (the forms or pages that is visible to the user) is separated from the data tier (the class or file interact with the database). This separation is done in order to improve the maintainability, scalability, re-usability, flexibility and performance as well.

A good explanations with demo code of 3-tier and 4-tier architecture can be read at http://www.dotnetfunda.com/articles/article71.aspx