What is a standard deck?

I recently began the Pokemon Online TCG. I noticed that I have a lot of cards in my collection that I'm not allowed to use in a "standard" deck.

I would assume that would be to stop usage of OP pokemon, but even my Wurmple card is not legal for a standard deck.

Can anyone explain to me what a standard deck is?

Standard refers to a specific format where only certain sets are allowed to be used. Right now that's Boundaries Crossed and every set released since its release.