SD cards have been very vital in the media world. They are placed internally of devices but are catigorized as external because they are so easy interchangable. They are widely used in cameras and recording devices. These devices usually come with very little internal storage and sd cards provide functionality and flexability, as they are easy to remove to get the data from them.
Blue ray disks are the new and current disk used today. They can hold much more data than the DVD which came before it. The expanded storage allows it to hold High Definition videos which have a larger file format than standard videos.
Solid state drives are a spin of regular hard drives. But they have no moving parts, they take the flash storage method. Because they have no moving parts the they work a lot faster than regular hard drives. This allows the users to access their data on the fly. However because of their great performance they cost a bit more than regular hard drives.