tiers

I just came across the following sentence: As the industry has moved from a three tier model to n-tier models, the object relational impedance mismatch has become more prevalent. But I can't find a concise explanation of the difference between three tier and n-tier. I know what three tier is, and I assume n-tier just adds one or more tiers. I'm just not sure what these additional tiers would be. If anyone has a short explanation or simply a good link, that would be much appreciated. The quote appears to be from this codeproject page. It also seems to do a pretty good job of explaining n-tier