When you talk about a person, you always say "a" if it's a direct object. For example, you wouldn't say "Odio los niños", that sounds weird, it should be "Odio a los niños". You wouldn't say either "Quiero Juan", you would say "Quiero a Juan"; but, of course, you should say "Odio los lunes" or "Quiero agua", because they are not people. (It's a little bit more difficult than this, but that's the general rule!)

La diferencia es la utilización de la preposición a al momento de hablar de una persona o un objeto Conocer a alguien: Conozco a la amiga de mi hermana. Conocer un lugar: Conozco Las vegas.