Both Nikon D3100 and FujiFilm S2950 have 14.0 MP resolution sensors but Nikon D3100's sensor is APS-C (23.1 x 15.4 mm ) and FujiFilm S2950's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Nikon D3100'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 FujiFilm S2950.
Below you can see the D3100 and S2950 sensor size comparison.
As seen above, Nikon D3100 has a 12.7x Larger sensor area than FujiFilm S2950. 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.