A two-tier architecture is where a client talks directly to 
a server, with no intervening server.  It is typically used 
in small environments (less than 50 users).

A common error in client/server development is to prototype 
an application in a small, two-tier environment, and then 
scale up by simply adding more users to the server.  This 
approach will usually result in an ineffective system, as 
the server becomes overwhelmed.  To properly scale to 
hundreds or thousands of users, it is usually necessary to 
move to a three-tier architecture.