The op amp is the thing represented by the triangle, which is what you originally asked. It consists of numerous integrated transistors and other components to form a basic analog amplifier block . It is normally configured with various passive and active devices to perform any number of functions, as shown in the Wikipedia article
In your referenced diagram, two resistors are connected so the op amp will function as an inverting amplifier with a gain of Rf/Rin. It is the resistor connections that make it an inverting amplifier with a fixed gain.
Is that all clear?