David,

>Say... What is actually the meaning of a "single-tier" application? I'm still not familiar with the programmer "culture" =)

Typical "monolithic" old-style Fox app is usually an example of single-tier. The data access, validation of input, process logic, and display of user interface are all intertwined inextricably to each other.

In multiple-tier design, the user interface is separated from the business logic and from the data access, all of which are handled by separate pieces of code or classes.

Those separate "tiers" can be combined all in one physical EXE and still be considered "logical n-Tier", but often are built into COM objects to produce "physical n-tier" or "distributed" applications.