A flashbang is just a certain type of grenade. It has a time delayed fuse, which you can learn more about in detail here:
http://science.howstuffworks.com/grenade2.htm Upon detonation, a flashbang releases over a million candelas, and a deafening 170-180 dB. The newest flashbang, the XM84 Stun Grenade, has a hexagonal body with holes in it that does not fragment upon detonation. The filler contains about 4.5 grams of a mix of magnesium and ammonium perchlorate. Most of this info can be found on howstuffworks.com or wikipedia.