Laptop specific Sound Cards are credit-card sized cartridges that connect via a PCMCIA or PC Card slot on the laptop. Typically, a laptop sound card will either use a CardBus connection or an ExpressCard connection. ExpressCard is the most common type and is found on most modern laptops. It is always recommended to be certain of the expansion slot type your laptop has to ensure that the sound card is compatible.
Because of their size, laptop sound cards are usually limited to one or two IN and one OUT mini-jack sockets. They are also a lot more discrete and portable than an external interface.