The BCG matrix is a portfolio planning model developed by the Boston Consulting Group in the 1970s. It classifies business units into four categories based on their market growth rate and relative market share: stars, cash cows, question marks, and dogs. Stars are high growth, high market share business units that require heavy investment to maintain their position. Cash cows are low growth, high share units that generate cash in excess of their needs. Question marks are high growth, low share units that require significant investment to build market position. Dogs are low growth, low share units that consume cash with limited prospects. The matrix is used to determine appropriate strategies for business units in each category.