As others have said, this is not a commonly spoken word, but is found mostly in poetry and writing, perhaps especially used in folk and children tales. I would use "acá y acullá" as the equivalent of "hither and yon".
As an aside, The RAE defines "acullá" as
adv. l. A la parte opuesta de quien habla. U. en contraposición a adverbios demostrativos de ...
"Aquí" and "Acá" have the same meaning. The same for "Allí" and "Allá".
We use "Aquí/Acá" when you're talking about a position near to you, or maybe your own position. In the other hand, "Allí/Allá" is for pointing a position far from you.
Aquí/Acá fue donde lo vi por última vez.
Esta silla de aquí/acá es de hace 5 años.
Vamos a comer allá/...
In Spanish there used to be a six-way proximal adverbial system:1
Far from both
However as the language evolved, these distinctions became fuzzier and many came to become synonymous in most circumstances:
aquí = acá
allí = allá
And in many American dialects:
ahí = ...
En Cuba aquí se usa mucho mas que acá (al menos en los pueblos y ciudades). Me atrevo a decir que el uso de acá se restringe a cuando aquí no es apropiado. Con esto afirmo además que no son iguales. Difícilmente diría "Muévete hacia aquí!" señalando a un lugar cerca de mí... diría "hacia acá" (pa'cá...). Noten que el sentido de direccion es lo que ...
Spanish has six demonstrative adverbs. In order from nearest to farthest, they are aquí, acá, ahí, allí, allá, acullá. English has just three: here, there, yonder. That's why it translated it as yonder, as they are both the farthest from the speaker (incidentally, being from the [US] South, I use yonder in regular speech and so while allí sits on a grey ...
Just a little addition to the existing good answers:
I believe that both allá and acá both originally were used in a similar way to the English thither and hither respectively (i.e. to there or to here), but this is not longer strictly required in modern Spanish grammar, as in modern English.
Currently, allá is typically used to mean over there, with a ...
As the RAE definitions compared in this other answer show, both words have the same meaning. However, from the questions and comments it is clear that in some regions one is preferred over the other.
By using Google trends (thanks @ukemi for this find), which shows what people are searching for, we can notice that:
aquí is overwhelmingly preferred in Spain
I was taught - by a Colombian - that aquí is more proximal than acá. That is, if something is in the general vicinity, but not at arm's length, one would use acá. If it was easily reachable, one would use aquí. (Exactly where the boundary lies is a bit fuzzy).
Is this the case in Colombia, but not elsewhere?
It's a word only found in books and cartoons.
Her's an example in "Alice in Wonderland":
Oye ven, ¿Por dónde está la reina?
A veces por aquí y a veces por acá. Pero como yo soy gente importante siempre entro por acullá.
Ahí = there
Allí = yonder
I prefer to think of the aquí/acá—ahí—allí/allá difference as being the same as 1st / 2nd / 3rd person. Aquí/acá is near me, ahí is near you, and allí/allá is near neither of us.
user983248 has a very good description of the five level distinction, although there is a sixth one, acullá, which is even farther away than allá. Of ...
In the North of Mexico we always say "aquí" even when we say "ven aqui" and we also use "allá" instead of "allí" but in the south of Mexico and most of south America "acá" is used. What is correct depends on where you are. By the way I'm in Monterrey very close to the US.