Concept:
Diuretics are drugs that increase urine output.
Step 1: Furosemide acts on the loop of Henle in the kidney.
Step 2: It inhibits sodium, potassium, and chloride reabsorption.
Step 3: This causes increased excretion of water and electrolytes.
Step 4: It is useful in edema, heart failure, and hypertension.
Step 5: Therefore, furosemide is classified as a loop diuretic.
\[
\boxed{\text{Loop diuretic}}
\]