what is a link farm?

link farms are the sites especially designed just to link with each other, because they link to each other hence have no quality or interest what so ever in visitors, many search engines like google and yahoo banned sites if found linked with link farms

It can't really hurt you from what I read. I just read that site linked to each other with total irrelevancy are just not a s prized highly as those that have inbound links that re releavant. For instants, if domainsocial.com an for example socialdomain.com were to fight for top ranks in google and socialdomain.com had more inbound links from sites about photography, he would loose out to domainsocial.com.

Yes relevency is another matter, you can achieve high rankings if you rank with your relevent sites, else they are no use, maybe they will get you rank for some time but in a long run you will not be able to mainain high ranking

Well if we follow Google than its not harmfull to us but some webmaster tell that it effect on our site. I don't know how can we control those inbound links.

I for one do not believe in this. For you see, in bound links are something that we personally cannot control. This would be a greater way for soporate sabotage wouldn't it? Anyways, if Google says it's fine, than lets listen to them. :-)

I never had any trouble and getting indexed like mad.
I think Google likes IPB forums.

Anyway I have submitted my domain with and without "www"
And still not having any trouble.