Client server is a 2-tier application. In this, front end 
or client is connected to
'Data base server' through 'Data Source Name',front end is 
the monitoring level. 


Web based architecture is a 3-tier application. In this, 
browser is connected to webserver through 
TCP/IP and webserver is connected to Data base 
server,browser is the monitoring level.
In general, Black box testers are concentrating on 
monitoring level of any type of application.

Some more to previous answer........

All the client server applications are 2 tier 
architecutures.
Here in these architecutre, all the "Business Logic" is 
stored in clients and "Data" is stored in Servers. So if 
user request anything, business logic will b performed at 
client, and the data is retrived from Server(DB Server). 
 
Here the problem is, if any business logic changes, then we 
need to change the logic at each any every client. 
 
The best ex: is take a super market, i have braches in the 
city. At each branch i have clients, so business logic is 
stored in clients, but the actual data is store in servers.
If assume i want to give som discount on som items, so i 
need to change the business logic. For this i need to goto 
each branch and need to change the business logic at each 
client. This the disadvantage of Client/Server 
architecture. 
 
So 3-tier architecture came into picture:
 
Here Business Logic is stored in one Server, and all the 
clients are dumb terminals. If user requests anything the 
request first sent to server, the server will bring the 
data from DB Sever and send it to clients. This is the flow 
for 3-tier architecture. 
 
Assume for the above. Ex. if i want to give som discout, 
all my business logic is there in Server. So i need to 
change at one place, not at each client. This is the main 
advantage of 3-tier architecture.

Very useful example. thanks to Uday..

In twoTier Application there are two cycle,interaction 
between client-server& interaction between server-user
In threetier Applcation seperate cycle (Client-server-user)

two tier architecture means in this layer database is 
seperated to the buisiness lozic and user interface
data is sharaed for multi users
three tier architecture:
three tier architecture means it is indudviduly seperated 
to the bl+Ul+database...
It is not affect to the another  layer