ExpressCard/34 is a standard for expansion cards used in laptops, allowing users to add functionality such as additional ports or storage. It measures 34mm in width and connects to the laptop's motherboard through a dedicated slot, providing a faster interface than older standards like PCMCIA. The ExpressCard standard supports both USB and PCI Express interfaces, enabling a wide range of devices to be connected. This versatility makes it popular for users needing to enhance their laptop's capabilities without requiring extensive modifications.