With a slur, you slide up to or hammer on to a note, and the emphasis is on that note. The slur can be used to accentuate it. With legato, all of the notes you play have the same emphasis, but you hammer on/pull off/slide between them.
Slur - A curved line placed above or below two or more notes of different pitch to indicate that they are to be performed in legato style
Legato - Smooth, connected.

Acorrding to that musical dictionary there pritty much similar, when you slur you slide or hammer to get to the next note and that is the same in legato aswel. Could be wrong though. Best to wait for some like Archeo Avis or Branny to answer.

Hope this helps!
A slur is (on guitar) when you play the first note then hammer on/ pull off for the second one.

Legato simply means to play something smoothly, whether that means hammer ons or picking every note is your choice. For example, if a large section of music had what looks like a slur on it then you wouldn't just play the first note then hammer on/pull off for bars - you would simply play it as smoothly as possible.
There's no difference on the guitar. The distinction is vital for other instruments, but not so much for guitar and certainly not in rock music (which dominates this website).