1) Relative keys have the same key signatures. The A minor key signature is the relative key of C major. E minor is the relative key of G major. All 15 major key signatures have a relative minor key.
2) Relative keys share a different tonic. The tonic of the relative minor is the sixth scale degree of the major scale. A minor is the sixth degree of the C major scale. E minor is the sixth degree of the G major scale.