Both Nikon D5000 and Canon ELPH 500 HS have 12.0 MP resolution sensors but Nikon D5000's sensor is APS-C (23.6 x 15.8 mm ) and Canon ELPH 500 HS's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Nikon D5000's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Canon ELPH 500 HS.
Below you can see the D5000 and ELPH 500 HS sensor size comparison.
As seen above, Nikon D5000 has a 13.3x Larger sensor area than Canon ELPH 500 HS. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.