A 16GB card can hold approximately 4,000 high-quality 12MP JPEGs or roughly 500-800 RAW files, depending on the camera's sensor resolution.
While technically possible for very short clips, it is not recommended. 4K video requires higher write speeds (U3/V30) and more capacity; 16GB would fill up in less than 10 minutes.
Yes, it fits any camera with a standard SD or SDHC slot. It is widely used in entry-level DSLRs and digital audio recorders.