http://www.wikilengua.org/index.php/a_%28preposici%C3%B3n%29

Gracias Barbara y Airam. One more question, is there other conditions where it's used?! or just in this case?!

When the Object Direct is a person we use the preposition "a" before it (Juan vio a su hermana - Juan saw his sister). When it's an animal we can use both, with "a" or without "a" depending on your relationship with the animal. But to tell the truth, at least in Spain, we normally use "a" for both people and animals. El perro persigue la gata (correct) El perro persigue a la gata (correct and I would say it's also more used)

También sería correcto decir "El perro persigue la gata" (sin la A)