What defines a Techcard?

So I've recently started watching more competitive Hearthstone and really enjoyed it ( excluding the terrible layout ).

One of the most fascinating aspects I found to be the fine differences between similar decks.

In that regard the casters often talk about Techcard-choices in decks. And I'm wondering what exactly qualifies as a Techcard?

Every classspell ? Only some of them?

Thanks for your answers in advance :)