You can load files, like photos and videos, on the SD card. You can install apps on the SD card. You can't transfer the SD card between devices. The SD card can be used in addition to your device's storage.

Secure Digital, officially abbreviated as SD, is a proprietary non-volatile flash memory card format developed by the SD Association for use in portable devices. 

Specification:-

Usage: Portable devices, such as digital cameras and mobile phones (including most smartphones)

Standard: SD Standard

Weight: Standard ~2 g; Mini: ~800 mg; Micro: ~250 mg

Block size: Variable

Extended from: MultiMediaCard

Write mechanism: Same as Read