As you probably know, in spanish you can change the order of the words and the meaning will remain the same. In the sentence: Juan drinks water the possible ways to say it is:
Juan toma agua
Agua juan toma
Juan agua toma.
There is no confusion since 'agua' is an object and there is no posibility that the water driks Juan.
But when the object (or second noun) is not an object and the verb can induce the posibility of a confusion or missunderstanding the preposition 'a' is needed.
In the sentence: Juan looks for Pablo.
Juan busca Pablo
Juan Pablo busca
Pablo Juan busca.
It is impossible to determine who is carrying out the action, for that reason 'a' is needed because 'buscar' is a "verbo transitivo". So,in all the sentences:
Juan busca a Pablo
a Pablo Juan busca
a Pablo busca Juan
All of them point out that Juan is looking for Pablo and not the other way around.
Adding an 'a' will aid you to rearrange the whole sentence with out inducing confusion.
A way of know if a verb is "transitivo" is by looking at its definition in a dictionary. Ther will be a small tr. .
But as a trick: if you feel that there is a chance of confusion, this is, if the "object" can carry out the action aswell, add the preposition 'a'.
As a final comment, "busca personas" refers to an order "Busca personas!" but obviously without the exclamation mark. :P