Hello Larry
The SD Card is an abbreviation of the full name, which is Secure Digital Card.
New SD cards can read & write faster then the original cards (Size is immaterial) and were developed because other devices, (SD Cards are used in a multitude of products) demanded faster memory. A Good Example is the Digital Camera, which in the old days took a few seconds to copy the data to the card before another picture could be taken, were as with modern Digital Cameras, pictures can be taken at 4, 5 or more times per second and so requires considerably faster memory.

Bill