You could find the answer in the following link: http://www.wordreference.com/es/en/translation.asp?spen=a

1. With verbs of movement like ir(to go) YO VOY A LA IGLESIA=I go to the church 2. When the direct object of a verb is a person or animal, not a thing. PERSON/ VISITO A MIS ABUELOS=I visit my grandparents THING/VISITO EL MUSEO(no a)= I visit the museum 3. To indicate the indirect object/to whom something is being done YO LE DI UN REGALO A DAVID=I gave a present to david 4. To indicate how something is cooked or done POLLO AL HORNO=chicken cooked in an oven POLLO A LA PARILLA= chicken cooked on a grill

sometimes is the same of ''to'' in english..