To be a solution, a mixture must be homogeneous--its components must be uniformly dispersed and separable only by chemical means. There are two parts of a solution, the solvent and the solutes. The solvent is what we call the major component and a solvent is one of the minor components. Solutions can be made between many phases of matter. For example, salt water is a solution of a solid solute (NaCl) in a liquid solvent (water) and air is a solution of a variety of gaseous solutes, including oxygen, in the gaseous solvent nitrogen.
To understand solutions we must first understand their compositions. To do so, we need to develop a quantitative way to talk about the concentration of a solution. Many different units of concentration have been developed. Mass percent is the mass of solute divided by the mass of the solution. Mole fraction is the number of moles of solute divided by the total number of moles of solvent. Molarity, perhaps the most common concentration unit, is the number of moles of solute divided by the number of liters of solution. Molality is the number of moles of solute divided by the number of kilograms of solvent. Normality is the number of molar equivalents of solute per liter of solution.
In addition to a discussion of the concentrations of solutions, we will briefly touch upon the ways in which solutions are made, including dilution.