A tie and slur look the same but they are telling you different things based on the context. A tie connects two of the same note and you hold that note for the sum of the two tied notes. For instance, if you have a half note tied to a quarter note you would hold that note for 3 beats (half note+quarter=2+1=3). A slur connects two or more notes and indicates that you play those notes connected or, in Italian, legato. Slurs can also indicate the length of a phrase.