If I use a JSP for presentation and a servlet for business logic and store the data in a database - this is a 2 tier architecture right?
My understanding of the concept tier is that it relates to the physical structuring of the application. In the above example the JSP and servlet would reside in the same place (web server) - this would mean I only have 2 tiers: Web Server and Database. Sound right?
I guess it depends if you consider the "Client tier" in your architecture or not. This would normally be a browser thin client - but it can definetely be considered as a tier.
Client tier --> Web/Business logic tier --> Data tier
Tiers can be either physical (as you suggested) or logical. For example you could have a 3-tier architecture at a high level (physical layout) and a closer look may reveile further logical tiers such as web/business/persistence tier on one server. Read into n-tiers also.