Since some folks seem to be using the two terms interchangeably, this thread is here to explain the difference.
A check is a Pokemon that can defeat another Pokemon in a specific situation, but in some other situations can not. A good example is Landurous against Raichu. Though Landorous can defeat a raichu once it is in, if it switches in it's risking taking a Surf to the face and fainting. This makes Landorous a check, as it can only beat Raichu if Raichu doesn't use Surf.
A counter is a Pokemon that can kill in almost any situation, bar hax or very strange and unreliable sets. A good example is Rotom H vs. Genesect. Genesect can not do anything to harm Rotom H, Ice Beam, Thunderbolt, Flamethrower, and Bug Buzz are resisted. U-turn can allow another pokemon to come in, but Genesect itself can do nothing, so Rotom-H is a reliable counter.
Please keep these in mind as you're writing.
