Why do programs, like Yahoo messenger or firefox need to act as servers?

Question

I mean, why does a program have to act as a server to go online...when it works when its not acting as a server? What does it mean?

Answer 1

Well thats because they are servers...
Yahoo pager is a server to yahoo. If you remove it how can yahoo connect to your computer uploading your contact list?

Firefox, because when you brows the net you entering ftp web pages meaning you are like in another computer. Read an artical under ftp if you want to, http://www.ftpplanet.com/ftpresources/basics.htm

Answer 2

A program doesn't necessarily have to act as a server. It can act as a client...

Perhaps you're associating the clients sending data upstream as server behavior. Not quite. Client and server don't mean that. Yes, clients will have to send data upstream to a server. Say you wanted to view a website. Well, if you want to download the website, you have to tell the webserver who you are, what webpage you want, what protocol you want to use, etc. Communication is a two way street, and data will be sent both ways.

Client/Server really refers to the architecture and what role something plays. It is not what direction of data they primarily send. For example, a P2p client can be sharing a lot of files. It is still a client. Something like a distributed computing server could spend most of its time downloading processed data.