Gheebah (backbiting, gossip) means that you mention something about your Muslim brother which he does not like, such as his faults. This is haram (impermissible), no matter by what means it happens or whatever form it takes. It is haram according to the consensus of the Muslim scholars. Criticizing people, if the aim is to expose their faults, is gheebah and is among the “evil uttered in public” that Allah does not like [4:148]. But if the intention is to put people off a person’s evil action that is forbidden according to sharee’ah (Islamic law), to cause a person to stop doing that, to combat someone’s bid’ah (innovation) and warn people against it, or to expose something false that a person has said in front of others in discussion groups or newsgroups on the internet or elsewhere, then this is permissible and is not the gheebah which is forbidden. Indeed it is an act of worship that brings a person closer to Allah, or it is an obligation upon whoever is able to do that. The salaf (pious predecessors) condemned innovators and critiqued the narrators of reports, in sincerity to Allah and His Messenger (peace and blessings of Allah be upon him). What is important is to look at the aim and intention.
And Allah knows best.