What does "receptor convention" mean in electrical circuits? What do the phrases "passive sign convention", "receptor convention", and "generator convention" mean with respect to electrical circuits, particularly for capacitors?
 A: I think this is really about which way you count current and voltage to be positive. For every element in a network you can define a a current and a voltage. If voltage and and current point the same direction, it's called "receptor". If they are in opposite direction its' called "generator". The most common convention is  to use "receptor" for resistors, capacitors and inductors and "generator for sources". 
It's a convention that simply provides that "most numbers come out to be positive". The math works either way and it's simply a matter of convenience.  
See also http://en.wikipedia.org/wiki/Passive_sign_convention
A: As Hilmar pointed out, this is a convention telling you in which way to count the current and voltage of a electric dipole.
Active components, such as generators, generally produce power. What it means physically is that it produces energy for the charges (let's picture positive charges for simplification), to go across a gain of potential. 
In the picture below, I have taken all currents and voltages to be positive. For instance, $U_0$ being positive means that the electric potential is higher at the top of the generator compared to the bottom. Because the electric current also points from bottom to top, it means that (positive) charges are going in that direction in the circuit. But because higher potential means higher energy (for positive charges), this costs some energy, which is provided by the generator. The rate at which this energy is provided by the generator is given by the power formula $P_0 = U_0 \times I$. You see that this convention for choosing the direction of current and voltage is adapted to generators, as it leads both of them to be positive, and also means that when using the $P = U \times I$ formula we are computing the power provided by the generator to the circuit, as it is expected for a generator. Hence the name "generator convention".

In contrast, all other elements in this picture have the current and voltages pointing in opposite directions. This means that they are in "receptor convention". In that situation, receptor convention is indeed the most adapted convention, because it leads to all quantities being positive (if the dipole doesn't provide any energy to the charges, then the natural way for them to go is from higher electric potential to lower electric potential, meaning that the voltage across the dipole will point in the direction opposite to the current). Receptor convention also means that when using the power formula $P_R = U_R \times I$ or $P_C = U_C \times I$, we are computing the power dissipated by either the resistor or the capacitor, which makes sense because since neither of those have the capacity to create energy, they can only receive it.
So in short the most convenient way to go is: 


*

*generator/active dipole -> generator convention -> voltage and current pointing in the same direction -> $P = U \times I$ is the generated power

*passive dipole (resistor, capacitor, inductor, etc...) -> receptor convention -> voltage and current pointing in opposite directions -> $P = U \times I$ is the dissipated power


Maybe you know some laws connecting the voltage to the current for certain components such as Ohm's law for a resistor ($U_R = R \times I$), the capacitor's law ($I = C \frac{d U_C}{dt}$), or the inductor's law ($U_L = L \frac{d I}{dt}$). All these laws are to be understood as a relation between the voltage and the current of said dipole in receptor convention. You can use the reverse "generator convention" even for resistors and capacitors, but this is highly non-standard as it will lead to an extra minus sign in all the previous formula, which is not convenient.
In the end, all of this is a matter of convenience. We choose the convention that is most fitted for each dipole so that all quantities are positive. If you stick with "generator convention" for generators/active components (meaning defining the current and voltage in the same direction), and "receptor convention" for all passive components (meaning defining the current and voltage in opposite directions), such as resistors, capacitors or inductors, you will be fine 99% of the time. Hope this makes sense.
