Spike is a creature type found in the Tempest block. Most Spikes are green, 0/0 creatures with "[This] comes into play with [a number of] +1/+1 counters on it" and "2, Remove a +1/+1 counter from [this]: Put a +1/+1 counter on target creature." The one exception to this is Spike Cannibal.
Spikes are depicted with a slug-like body with two rows of pods down either side of the back and a pair of pincers on the face.
Tempest had only one Spike, the simplest, Spike Drone. It served as a preview of sorts of the rest of the Spikes yet to appear in that block.
Stronghold had five Spikes. The two commons, Spike Worker and Spike Colony, have no additional abilities beyond those described above. The other three, Spike Feeder, Spike Soldier, and Spike Breeder, each have an activated ability that involves removing +1/+1 counters from it as a cost.
Exodus had four Spikes. Spike Rogue, Spike Weaver and Spike Hatcher each introduced a new use for their +1/+1 counters. Spike Cannibal is the big exception to the nature of Spikes. While still a 0/0 creature that comes into play with a +1/+1 counter, Spike Cannibal is black and lacks the Spike's natural "donate a counter" ability. Instead, Spike Cannibal has a comes into play triggered ability that moves all +1/+1 counters onto it, effectively cannibalizing other spikes and +1/+1 counter-dependant creatures.
Spikes have served as inspiration for the creation of other abilities in later sets, including Modular and Graft.