Trigonometry is the measurement of triangles. It was first developed by Greek mathematicians like Hipparchus in 150 BC to compute missing lengths and angles of triangles using ratios like sine, cosine, and tangent. The six trigonometric functions are sine, cosine, tangent, secant, cosecant, and cotangent. Trigonometry involves identifying the sides of a triangle - the hypotenuse, opposite, and adjacent sides - and using trigonometric ratios to calculate unknown values. It has applications in fields like navigation, surveying, and physics.