Terraria – What do I do with all this money

They are Demon Altars which are not monsters - hence you cannot get the tooltip by hovering over them.

You use them to craft some 'boss' summoning materials Slime Crown, Suspicious Looking Eye and Worm Food, as well as creating the stronger pre-hard mode weapon Night's Edge.

Demon Altars can only be destroyed with Pwnhammer or Hamdrax after Wall of Flesh is destroyed. Trying to destroy Demonic Altar before defeating Wall of Flesh, you will take half your current health in damage minus armor, no matter the tool. Pwnhammer is acquired by defeating Wall of Flesh.

Destroying a Demonic Altar will "bless" your world with decreasing amounts of ore, starting with Cobalt, then Mythril, then Adantium and then starting the loop again. This means that it will convert random blocks into whatever ore it is blessing your world with.

It will also place Ebonstone/Pearlstone in random locations underground, allowing Corruption/Hallow to spread faster and from multiple locations.

Source: Terraria Wiki

You shouldn't use it as a building material because the presence of a lot of Ebonstone causes the sky and music transition seen in the Corruption, and allows Corrupted enemies to spawn.

What you could do with it:

• Place it in your inventory's trash slot to get rid of it; use Shift-Click for maximum efficiency.
• Place it, then purify it to get more Stone, if you happen to be short on that (which will never happen because half the map is Stone).
• Store it on the assumption that it does something useful at some point in the future.
• Use it to lock a passage to players who don't have a Nightmare Pickaxe or higher yet. Then again, other materials, such as Obsidian or Hellstone, would be better suited for such purposes.
• Use it to build a Corrupted enemy grinder for infinite Rotten Chunks.

As of patch 1.1, there are also a few more "real" uses:

• You can combine it with Demonite Ore to make Demonite Bricks, which are a feasible building material.
• After unlocking hardmode, the Corruption can spread via Stone Blocks as well, so you can get an Underground Corruption going by planting a few Ebonstone Blocks, then waiting for them to spread.