There are two theories that explain chromatography: plate theory and rate theory. Plate theory, developed in 1941, views the column as divided into theoretical plates where analytes completely equilibrate between the stationary and mobile phases. Rate theory, proposed in 1956, accounts for the dynamics of separation. Greater separation occurs with more theoretical plates and smaller plate height. The number of theoretical plates can be calculated using methods like half-height or USP, and depends on factors like column length, particle size, and retention time.