parallel filters vs series filters

So I have been having trouble understanding the difference of Rc filters that have the capacitor in series with the resistance and Rc filters that have the capacitor parallel to the resistance . To make this more specific , why are both low pass filters? Can all filters be made in parallel and in series?

• The voltage on a capacitor can't change "suddenly". So it will block any fast (high frequency) signals. That's the intuition. The rest is a math. Dec 1, 2017 at 16:30
• They are not both low pass filters. The upper circuit is a high-pass filter. Dec 1, 2017 at 16:51
• Check the circuit at the two extremes. Effective resistance of a capacitor at 0 freq is infinite. Conversely, at infinite freq, the capacitor looks like a short. Dec 1, 2017 at 16:53
• Convert both filters to Z impedance and you'll see that their different Dec 1, 2017 at 16:55

But the fact the capacitor is labelled $\rm C_f$ suggests this filter might be used in the feed back path of an op-amp circuit.