Solvents are used to dilute colour, as well as for cleaning brushes and palettes after a painting session. Solvents made for artists are intended to be fully volatile, meaning that upon evaporation from the paint mixture, they leave no residual matter behind.
Hardware/DIY grade solvents, though lower in cost, are not refined to the degree required for artists, and will often leave a tacky painting surface where used, and a paint film that won't fully dry.
For details on when to use each solvent see our usage chart.