The Setblock Command

The /setblock command is a basic command used in simple building or the changing of one block.

Overview
The /setblock command changes a certain block into another block. For example, typing /setblock 100 10 100 tnt will change the block at 100, 10, 100 to TNT from whatever it was before.

Syntax
/setblock   

Explanation
The /setblock command sets the block at the coordinates written at the first three values. (Press F3 to see coordinates.) The block value is the block that the block at the previously specified coordinates changes into.

Trivia

 * This is pretty similar to the Fill command.