Young tableaux are combinatorial objects used in representation theory and algebraic geometry. They consist of a grid filled with positive integers that increase across each row and down each column. Each number must be unique, and the arrangement reflects a specific partition of a number. These tableaux help visualize and analyze the structure of symmetric groups and Lie algebras. They are particularly useful in the study of Schur functions and Young diagrams, which represent partitions graphically. Young tableaux can be standard, where the numbers are filled in sequentially, or semistandard, allowing for repeated entries while still following the same increasing order rules.