This block when activated with the detenator will destroy all blocks in a 5x5 area around the block including itself, somtimes when using this on servers with lag the blocks may or may not be destroyed and the TNT may or may not be destroyed, none of these blocks function and are just 'Ghost Blocks'.
TNT is also one of the most common tools for griefing and killing as it destroys a large area while killing any player in range, a max of two TNT may be placed at once before having to gather more metal/ore as each requires 100 to place making this the most expensive block in the game with the beacon and bank following at 50 each.
Not all blocks are destroyed by this, so I will list what blocks are not destroyed by this:
TNT can destroy all player placed blocks with the execption of the Beacon and Bank blocks hence them being 50 each, this makes using bank blocks a valuble tool when building a base as the only way to destroy these are to be on the same team and as a engineer that uses the deconstruct friendly block tool.
Lava can be destroyed with TNT although unless you remove all of the surrounding lava it will flow back into the hole you cleared, this makes hidden bases under lava a very real possibiltiy although a costly one. Both the Gold and Diamond blocks, this means that you may use TNT to clear the area around the ores without fear of destroying them.