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 sense of remoteness beyond that of allí
. It should be noted however that acá
is used rather rarermore rarely, and when it is, it still tends to retain its original meaning of hither
.