Capital Gains has its formats or varieties, in fact two of them called:
Short Term, and
Long Term
Simply said, its based on assets I sell, which are Short Term Assets (call as Short Term Capital Asset) and Long Term Assets (call as Long Term Capital Asset).
This long and short is just a period -based distinction, means how long I hold them. The distinction is important, as Long Term Capital Assets are taxed at special (normally its lower) rates than Short Term Capital Assets.