The verbs sapere and conoscere both mean "to know," but have different implications. Sapere means "to know," "to be able to," or "to know how to." Take a look at these examples with the io form of sapere:
So la lezione. (I know the lesson.)
So cantare. (I know how to sing; I am able to sing.)
Conoscere means to know, to be acquainted with, or to make the acquaintance of:
Io conosco Francesca a casa di Giuseppe. (I met Francesca at Giuseppe's house.)
Non conosciamo la città. (We don't know the city.)