Crystalloids are solutions that contain small molecules, such as salts and sugars, which can easily pass through cell membranes. They are commonly used in medical settings for fluid replacement and hydration. Examples include normal saline and lactated Ringer's solution, which help restore electrolyte balance and maintain blood volume. These solutions are typically classified into isotonic, hypotonic, and hypertonic types, depending on their osmolarity compared to blood plasma. Isotonic crystalloids, like 0.9% sodium chloride, are often used for general hydration, while hypotonic solutions can help with cellular hydration, and hypertonic solutions may be used to treat specific medical conditions.