Take one minute of video. Look at the file size. Now how many of these 1 minute files can you fit onto an 8GB card in your particular format and camera/device.
SMS stands for Short Message Service, which is the formal name for text messaging. It's a way to send short, text-only messages from one phone to another. These messages are usually sent over a cellular data network. That's not always true, though, as in the case of iMessage below.
A multimedia message is message sent or received on your mobile device that contains a picture, video or other multimedia file attached. Group messages are also considered multimedia messages. Multimedia messaging is also commonly referred to as MMS or picture and video messaging.