Ser = Sees, happens to spot. "Han är på promenad och ser en ko." "He is on a walk and sees a cow."
Tittar = Looks. "Han tittar inte." "He is not looking." (i.e., he is for example covering his eyes.)
Tittar på = Watches, looks at something. Must have an object, though it can be an implied one. "Han tittar på tavlan." "He is looking at the painting." "Ebba spelar spel och Kalle tittar på." "Ebba is playing games and Kalle is watching ("Ebba playing games" being the implied object)."