Differences between parallel resistor-capacitor filters and series resistor-capacitor filters
It's not clear what exactly you mean by series versus parallel resistor-capacitor filters.  Posting a schematic of each would clarify this.

You probably mean series is something the signal passes thru, and parallel is something that works as a shunt.  Note that the same thing that is a low pass filter in series is a high pass filter as a shunt, and vice versa.

Basically a capacitor will block low frequencies and short high ones.  If you put it in series with a signal then it is a high pass filter.  If you put it accross a signal, it will short the high frequencies thereby making a low pass filter.  The value of the capacitance and the resistance it is working against tell you the -3dB rolloff frequency of the filter, whether high pass or low pass.  This frequency is:


  \$ f = \dfrac{1}{2 \cdot \pi \cdot R \cdot C} \$


When R is in Ohms and C in Farads, then f is in Hertz.