This document defines acids and bases and their key characteristics. It explains that acids lower the pH of solutions below 7 while bases raise the pH above 7. Acids can donate protons while bases can accept them. Strong acids and bases ionize completely while weak ones only ionize partially. Indicators are used to detect the presence of acids and bases based on color changes. Different models like Arrhenius, Brønsted-Lowry, and Lewis are presented to describe acids and bases. Examples of common acids and bases along with their pH levels are also provided.