What's the difference between solids and liquids?

The molecules of solids are more tightly packed therefore solids are a matter with more definite shape and hardness. Meanwhile liquids are made up of particles loosely packed so being all liquidy and less defined in terms of shape.

also important! liquids take up the shape of the space they're in

Solids do not

Liquids don't have a definite shape, but solids do.