To build, type in chat:
addblock <texture name> <x starting pos> <y starting pos> <z starting pos> <length> <height> <width>
For example:
addblock stone 0 0 0 1 7 1
Would make a stone pillar from the center of the map 7 blocks high. (and 1 wide and long)
To remove blocks, right click them if they're yours. (or if you're an admin)
To get info on a block, left click it.