A sparse matrix is a matrix with mostly zero elements. Storing a sparse matrix using a 2D array wastes space since it allocates memory for all elements including zeros. Sparse matrices can be represented more efficiently using two methods: triplet representation stores only the row, column, and value of non-zero elements; linked representation stores non-zero elements and their indices linked together to save space. The triplet representation example shows storing a 5x6 sparse matrix with 6 non-zero elements using a single array that lists the row, column, and value of each non-zero element.