Occasion where members of the press are gathered together to receive information about a particular subject. This is a situation that is positive for both parties: the person (or company) that has something newsworthy to tell gets to talk to many journalists at once , the press gets good stuff to publish and the chance to ask questions.
Sometimes though, a press conference is held when somebody else than the information giver has decided a press conference would be useful (in the case of politicians or film stars this usually involves a scandal that needs explaining). In such a case the press are often experienced as a crowd of nosy bastards asking difficult and impertinent questions.