Diffraction

Diffraction is the slight bending of light as it passes around the edge of an object. The amount of bending depends on the relative size of the wavelength of light to the size of the opening. If the opening is much larger than the wavelength of the light, the bending will be almost unnoticeable. However, if the two are closer in size or equal, the amount of bending is considerable, and easily seen with the naked eye.

Interference

 When two waves travel in the same medium, the superposition of the waves resulting in a new wave pattern and this phenomenon is called interference.

Single slit and Double slit

In a single slit diffraction, light spreads out in a line perpendicular to the slit.

But in a double slit diffraction, light diffracts when passing though the slits, but the light from those then interferes to produce an interference pattern on the screen.

The light is spread out in a line, like in the single slit, but here there is interference producing regions of constructive (bright fringes) and destructive (dark fringes) interference and a very bright spot at the centre of the screen, called the central maxima.

So, looking at diffraction only, there is no difference between single slit and double slit because in both cases diffraction happens.  But in a double slit there is diffraction and interference among the diffracted rays.