Solvents are liquids that dissolve other things. You all use them - stain removers, nail polish, etc. are solvent-based [[http://en.wikipedia.org/wiki/Solvent]]:
A solvent (from the Latin solvere, "loosen") is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature. Common uses for organic solvents are in dry cleaning (e.g. tetrachloroethylene), as a paint thinner (e.g. toluene, turpentine), as nail polish removers and glue solvents (acetone, methyl acetate, ethyl acetate), in spot removers (e.g. hexane, petrol ether), in detergents (citrus terpenes), in perfumes (ethanol), and in chemical synthesis. The use of inorganic solvents (other than water) is typically limited to research chemistry and some technological processes.
There are essential for most industrial and academic chemical reactions. Normally we dissolve the reagents in one or more solvents. Then we have to remove the solvents by filtration or distillation/evaporation. Then we may need to purify the result and this can be either by crystallization (again from a solution) or by chromatography (eluting the compound though a column).
All this has an impact. Many solvents are flammable, many are toxic; many are both! If they escape to the environment (e.g. by being washed down the drain) this can have serious effects.
And they are expensive to make, so we often try to recover them. All of this takes energy.
Some solvents are fairly friendly (e.g. water); some are very unpleasant (hydrofluoric acid, carbon tetrachloride, etc.). Each solvent can be given an index of acceptability. If we multiply this by the amount used that is an indication of how (un)friendly the process is.