How does a tie differ from a slur?

A slur indicates two or more notes that are to be played without any break, such as on a wind instrument not re-tonguing, so that the notes are played legato (smoothly).

A tie, is when two or more successive notes (of the same pitch) are held, unbroken, for their total combined time value. For instance, if a half note was tied to a quarter note, the duration of the tied note would be 3 beats.