What 's SD Memory Card ?

Today one colleague ask me what is SD Memory Card ? first i am so astoundment, i though all the people know SD Memory Card infomation. then i explain it to him, SD Card used to storage photos on digital camera or camcorder , other way it means SD card is a space to storage things. why he didn't know SD Card, he said he have not used it. oh, my god! never used then have nothing about this thing? i have never used a laptop, but i know some seireis about all the brands, such as HP Pavilion DV1000, and i know the battery for HP Pavilion DV1000 ( high Capacity can used more times than the low's).
Maybe most of things we have never used, but some useful infomations we must to know.