If said with malice, this is harrassment. A proper holla'ring should make the holla'ree feel good and complimented, not embarrased or threatened. Regardless, the act's success and harmlessness depends on both parties and their own histories, and so it's not usually recommended.
Darrell: "Girl, I just wanna let you know... the back of YO head is REDICKIRLOUS sic."
Joey from "Friends": "How YOU doin'?"
Examples of the word's use:
"OMG, did you see that? I just got holla'd at! Holla!!"
"Um.. I think I got holla'd at... I feel kind of awful now because I've been severely harrassed in the past so... this kind of sucks"
"Oh man, he totally holla'd at that girl! Wait, she's smiling? They're walking away holding hands? Good for him. He's really come out of his shell."
"I holla'd five times today, and it was really awkward having to tell the girls interested on following up on that that I was just interested in complimenting strangers."