The difference is the way they get their energy input to work against entropy, basically.
Desalination plants often use reverse osmosis, in which pressure is used to force water molecules through a semipermeable membrane that does not allow the larger, solvated, salt ions through. (Although an individual Na+ or Cl- ion is smaller than an H-O-H molecule, these ions in solution are surrounded by a solvent "cage" which is quite tightly bound to it and makes their effective size a lot bigger.) Alternatively distillation is used, often under reduced pressure to permit boiling at a relatively low temperature. (Boiling greatly increases the surface area for evaporation, accelerating the process, due to the surface area of the bubbles). The very large desalination plant at Jebel Ali, in the UAE, used that method, I remember, from when I lived in Dubai in the 1980s. It employed waste heat from the Jebel Ali steam power station - quite clever.
A solar still relies on heat from the sun, rather than an artificial energy input. So the energy input is in a sense "free" - and non-polluting, of course - but you are limited to the energy density of sunlight. Solar stills also have to rely on evaporation at atmospheric pressure, which further constrains their capacity.