how to realize alert in real time

hello, every body here,
I am a beginner of Bro, I have a question,hope some one can help me.Thanks.
According to some literature, "bro can make intrusion announcement in real time", but when I try to run bro, I don't find how to realize this function, I only can create some logfiles. And, if it do this as said, what is the form of alert?
Thank you very much.
sara young