Spikes are essentially walls that inflicts damage upon contact with players (but not mobs), as well as upon players who attack it (although it does less damage for the latter). Like other structures, spikes inhibit the movement of mobs and players.
Spikes deal more damage as they are further upgraded. They are often used to build bases to keep enemies away. Spikes do not harm mobs, the player that placed it and his teammates. Spikes can be damaged with pickaxes or amethyst weapons, but are destroyed most quickly by hammers. Like all other structures, a spike can only be placed once every 0.8 seconds.
Here are the pages for each type:
See also :