The "TF" in TF memory card is an abbreviation for TransFlash, and it is the original name for what is now commonly referred to as a microSD memory card. The SD stands for "secure digital," and these cards are the small cards used for extra storage in small mobile devices.
As of 2014, TF memory cards, or microSD cards, are physically the smallest memory storage cards available, measuring 15 by 11 by 1 millimeter and weighing just half a gram. These cards are used in most cell phones and smartphones. They are generally a bit more expensive then regular SD cards with the same speed and size class, but they are otherwise the same. However, each device dictates the type and size of card needed.