Are Aquí and Acá equivalent terms or do they vary slightly in the way they are used?
Aquí and Acá
We use the word "Aquí" to express here, but to specify where it is, for example: " el lápiz está aquí". The pencil is here (in a specific place). We use "Acá" for " El lápiz está acá! o "el lapíz esta por acá". In English you say "the pencil is over here". It is almost the same, but Aquí is used to show the exact location and Acá is used when the thing/person is near to us but we don't really know where it is.
I hope you understand, my english is not so good yet
I found this to be an excellent discussion on the topic http://community.livejournal.com/linguaphiles/3158725.html Edit: I give up, I can't make links work.
Acá is more general that aquí. Acá is like "over here", aquí is closer to where the speaker is at. Like "right here". You may sometimes here a hispanic mom or dad in the store tell their child "ven acá", telling them to "come over here".