Bombs are a siege weapon that can be used to destroy virtually any player-placed building or vehicle. They resemble a stereotypical black sphere.
When a Bomb catches on fire, it plays a loud beeping sound, flashes for 5-7 seconds, emits a Flame particle and then detonates; destroying and burning any blocks around it in a large, circular explosion. The explosion can also instantly kill any player that happens to get caught up in it.
Here is a video that shows exactly how bombs tend to operate if you're unsure.
Tips and Tricks
Bombs play fairly loud and distinctive sounds, use this to your advantage to locate a bomb if it hasn't already gone off.
Bombs can also be extinguished like any other burning block, this gives you a chance to disable and recollect a bomb before it goes off.
You can use welded items (Cactus, Coconut or Carved wood ) to make a 'sticky bomb', simply drag the Bomb onto the item after placing it. This can be helpful if you're trying to place it on moving objects (fast boats) or need a more precise placement.
Bombs take a good portion of time to recollect with your Forage tool, generally making it a bad idea to attempt to recollect a bomb near hostile players.
The radius of a Bomb's explosion can actually extend past a singular wall, which means any players or objects up against the wall will be obliterated. Use this to your advantage when breaching enemy forts.
Sometimes, chunks of Stone or Sandstone can be left behind after you blow up a structure, this can potentially thwart enemy players from patching the hole you just made (or force them to make an uneven structure)
You can chain Bomb explosions into other bombs if you position them correctly, this can lead to extremely devastating fort demolition when paired with the 'sticky bomb' trick to make sure they don't get thrown out of the way.
Bombs also synergize very well with Catapults, simply load a flaming Bomb into it and watch it fly!
Try to place bombs in hard-to-see spots (or put Foliage Thatch above it), as hostile players may attempt to drag it away from the designated target!