Far and away the most common memory cards used in consumer electronics are replaceable Secure Digital (SD) cards. SD is used by over 400 brands in thousands of models, in products ranging from cameras and laptops, to video game consoles, mp3 players, and phones.
SD cards are broken down into four speed class ratings: Class 2, Class 4, Class, and Class 10. Each numerical designation corresponds to the card’s Read/Write speed. A Class 2 card, for example, transfers 2 megabytes per second, a Class 4 transfers 4 MB, and so on. Transfer speed is important for a variety of reasons, but it’s a particularly pressing matter in instances of live recording; a camcorder (or video equipped smartphone), for example, needs to write image data at a certain speed, lest the recording lose fidelity. Thankfully, most contemporary devices will alert the user if the card inserted falls below its speed requirements.
SD cards come in three sizes—standard, mini, and micro—and are further broken down into four different functions: Standard Capacity (SDSC), High-Capacity (SDHC), eXtended-Capacity (SDXC), and Input/Output (SDIO). New devices tend to accept older iterations of SD cards, but old devices will rarely accept newer models. SD adapters are also readily available, which can eliminate compatibility issues based on SD family and size. Nonetheless, it's always best to consult your device's manual and specifications before purchasing a new card.