Acidic and basic solutions are characterized by their concentration of hydrogen ions (H+) and hydroxide ions (OH-) respectively, with acidic solutions having a higher concentration of H+ and basic solutions having a higher concentration of OH-. The pH scale, ranging from 0 to 14, measures the acidity or basicity of a solution, with values below 7 indicating acidity, values above 7 indicating basicity, and a value of 7 representing a neutral solution like pure water.