Percentage weight for weight - %w/w

This means the percent by weight of solute in the total weight of solution. Percent here is the number of grams of solute in 100 grams of solution.

For example, a 10% w/w solution of sodium chloride would be made by first dissolving 10 grams of sodium chloride in a solvent (most probably water in this case) and then adding solvent to a final weight of 100 grams of solution. (This is not 100 grams of solvent, but sufficient solvent such that the final weight of solution, that is, solute (sodium chloride) plus solvent (water), is 100 grams.

Percentage weight for volume - %w/v

This means the percent by weight of solute in the total volume of solution. Normally used where the solute is a solid.

For example, a 10% w/v sodium chloride solution would be prepared by dissolving 10 grams of sodium chloride in a solvent (again, say, water) then making up to a final solution volume of 100ml.

Percentage volume for volume - %v/v

This means the percent by volume of solute in the total volume of solution. Normally used where the solute is a liquid.

For example, a 10% v/v ethanol solution (in water) is 10ml of ethanol in 100ml of solution.