My mother uses this word a lot and being "born and raised" within 1 mile of where I live now in Florida, I have heard it a lot. Imagine my surprise when I found it in the dictionary.
hol·ler1
–verb (used without object)
1. to cry aloud; shout; yell: Quit hollering into the phone.
–verb (used with object)
2. to shout or yell (something): He hollered insults back into the saloon.
–noun
3. a loud cry used to express pain or surprise, to attract attention, to call for help, etc.
[Origin: 1690–1700, Americanism; var. of holla (see hallo)]
Alternate definitions include "a complaint or gripe" and a reference to "Appalachian Engllish". Since it referred to "hallo" I looked that up too. Very interesting word history!
hal·lo
noun, plural -los, verb, -loed, -lo·ing.
–interjection
1. (used to call or answer someone, or to incite dogs in hunting.)
–noun
2. the cry “hallo!”
3. a shout of exultation.
–verb (used without object)
4. to call with a loud voice; shout; cry, as after hunting dogs.
–verb (used with object)
5. to incite or chase (something) with shouts and cries of “hallo!”
6. to cry “hallo” to (someone).
7. to shout (something).
Also, halloa, halloo, hallow, hillo, hilloa, hullo, hulloo.
[Origin: 1560–70; var. of hollo, itself var. of earlier holla < MF hola, equiv. to ho ahoy + la there]
I would offer up that it does not always require shouting to be used. The phrase "Give me a holler" does not require the caller to actually yell. It simply is asking for someone to call another.
And yet, they don't always speak the same language we do. 
I guess I had it figured as more southern slang than a legitimate term.
