Is pyrex just tempered glass, or is there anything else in it?

What makes something pyrex instead of just glass? I am interested in buying some pyrex tupperwares, instead of plastic ones, to store leftover food and bring my lunch to work (I’m worried that the plastic is leaching toxins into my food…especially since I need to microwave my lunch to heat it up at work). Is pyrex the same as glass? And can I put the pyrex container in the microwave?