A fundamental problem in knot theory is determining when two distinct knot diagrams represent the same knot. This is traditionally addressed through the use of invariants such as the knot group, Alexander polynomial, and Jones polynomial, among others. While the knot group distinguishes prime knots, it is known to be incomplete as a knot invariant. In this talk, I will introduce quandles, an algebraic structure that provides a complete invariant of knots, and explain their connection to classical knot invariants. If time permits, I will also discuss their applications to virtual knot theory and quandle-based invariants derived from quandle cohomology and quandle colorings.