On a basic level, "a" is needed whenever you are referring to a human.

Yo veo al hombre.

Invito a John.

Pregunto a la profesora.

Side note: You do not need it if there is another preposition already present.

Yo habla con ella.

Camino hacia Robert.

Canto mi hermana.

There is a more advanced answer with lots of fun complex grammar stuff, but it explains WHY you need the 'a'. Let me know if you're interested.
Hope this helps! 

connect the verb with "a" when you are referring to people. No need to include the "a" when relating to things. 

Great, gracias!

Actually, the meaning is more or less the same. The more common use is: "Veo a un hombre y a una mujer" (Yo es facultativo). You can ask to a teacher of Spanish for a clear explanation. There's no problem if you say both anyway. Natives use often the first sentence. 

When to use a/al instead of el/la? I don't understand what do you mean in your question? 

Well even me that im a spanish nativa speaker, i dont really know, i think its more appropiated to say "a"  It isnt wrong if You dont use it but It also sounds better