An exact sequence is a sequence of mathematical objects, typically groups or modules, connected by homomorphisms, where the image of one homomorphism equals the kernel of the next. This property ensures that the structure of the objects is preserved and provides insights into their relationships. Exact sequences are fundamental in areas like algebraic topology and homological algebra. In an exact sequence, if you have a sequence of three objects A, B, and C with homomorphisms f: A → B and g: B → C, the sequence is exact at B if the image of f is equal to the kernel of g. This concept helps in understanding how different algebraic structures interact and can be used to derive important results in mathematics.