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.