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

I am not able to understand which elements are called as first tier, second tier & third tier & where they reside. Can they reside on same machine or different machine. Which tier reside on which machine? How we can identify a particular application as a 2 tier application or 3 tier application. Please explain with example

Solution

Wikipedia explains it better then I could

From the article - Top is 1st Tier: alt text