Minecraft (Pocket Edition)
Silverfish are found in Silverfish Blocks, although they are not that common. They are a Hostile Mob creature that will attack when provoked. They can be killed with any block, though most efficiently with a Sword.
Silverfish are found in the Extreme Hills Biome and in Strongholds. Typically Silverfish spawn from a Silverfish Block, which looks like a regular stone block. The block takes longer to mine than it normally would, and when the stone breaks a Silverfish spawns. If the Silverfish is left alone it will cause other Silverfish in the vicinity to Spawn or it will return back to the stone.
When in a Stronghold you will encounter a Monster Spawner generating Silverfish near the End Portal.
Usage / Purpose
Silverfish are a useless nuisance.
Drops / Yields
Silverfish do not drop anything at this point, and are themselves the drop from a Silverfish Block.
Silverfish are available in both Survival Mode (as a creature spawning from Silverfish Blocks), or in Creative Mode as a Spawn Silverfish Egg.
A useful battle technique is to elevate yourself up above the Silverfish and strike from above.