Single slit: a continuous line of light.
Double slit: a pattern of bright and dark fringes.
A single slit diffracts light only so the light is spread out in a line perpendicular to the slit.
The double slits both diffract light, but the light from those then interferes to produce an interference pattern on the screen. The light is spread out in a line like with the single slit but the interference causes the line to include regions of constructive and destructive interference. Where there is constructive interference the light is brighter. Where there is destructive interference the light cancels out to leave dark fringes.
Here is a video showing the interference pattern with a laser and double slits: