Saber
Use saber to say that you know a fact or information, or to say that you know how to do something. Facts and skills are common uses of saber.
Conocer
Use conocer to say that you know a person or are familiar with a thing. People and places are common uses of conocer.
The Personal “A”
When you use conocer to say that you know a person, you must use the preposition “a” before the noun referring to the person. This personal “a” must be used whenever a person receives the action of any verb (not just conocer).
Ex: Conocemos a Nina y a Roberto. (We know Nina and Roberto)
¿Ves a tus amigos frecuentemente? (Do you see your friends often?)