As per first part of yours question, in client server
application we have to install it &  then we can use it
directly without internet connection.
 but in the case of "yahoo messenger" though we have to
install it initially in our system & for every time using
this application we need web connection. thats why its web
application only.

I think ur little bit confused...

there are commonly 4 types
1.One tire 
2.Two tire
3.Three tire and 
4.N-tire appliations

In an architecture we have three Business,presentation and 
database layers.
1. In the first case the three layers include in one system 
ex: ur personal computer
2. In the second case the user will have the Business and 
presentatin layer and he/she collects the data from the 
database layer ie; from the server..
ex: Software institutions
3.In the third case the user will have the business layer 
and he/she retrives the data form the owner and he/she 
collects from the main database..
ex: Software companies.....
4. The same goes to the N-tire levels Here the users will 
be many as u asked Yahoo web site....
ex: Browsing centres.....

I think u understood clearly

any douts u can mail me to ajay.thomala@gamil.com