Two Tier,Three Tier and Multi Tier

The tiers refer to the number of "layers" in your system. A two-tier architecture could be where you have a client in layer 1, and the business logic (components and such) and the database access in layer 2. A three-tier architecture is where you split up client, business logic and database access into three separate layers.

2-tier example: A web page that takes the users input, processes it in the page and inserts it into the database.

3-tier example: Web page that takes the users input, feeds the input to a serverside component which processes it and finally commits a result to a database.

I hope this has made it a bit clearer.