ExpressCard is a standard for expansion cards used in laptops, allowing users to add new features or capabilities. It was introduced to replace the older PC Card standard, offering faster data transfer rates and a more compact design. ExpressCard slots can accommodate various modules, such as USB ports, eSATA connectors, or even additional storage. The standard supports two sizes: ExpressCard/34 and ExpressCard/54, with the numbers indicating the width in millimeters. This versatility makes it easier for users to customize their laptops according to their needs.