Actually, "coupling" is a proper description. The capacitor couples two circuits together by the common capacitive reactance of both circuits. The capacitor provides current at the rate to keep the voltage across it constant, so a change in voltage results in a change of current. The voltage change of the input signal causes a current change in the coupled circuit - except for DC, which is blocked.
Output capacitors also can act as DC blocks but they can also be used for filtering. Output caps in some tube circuits bypass harmonic frequencies to ground, before they reach the xfmr.
Bypass caps are used on the cathode of a tube to help with self-biasing the circuit. The voltage across a cathode resistor will vary with the signal voltage across the grid. As the voltage across the resistor varies, so does that of the grid and the bias. If the cathode resistor is bypassed with a capacitor, the voltage across the resistor remains constant which keeps the bias voltage constant.
Output capacitors also can act as DC blocks but they can also be used for filtering. Output caps in some tube circuits bypass harmonic frequencies to ground, before they reach the xfmr.
Bypass caps are used on the cathode of a tube to help with self-biasing the circuit. The voltage across a cathode resistor will vary with the signal voltage across the grid. As the voltage across the resistor varies, so does that of the grid and the bias. If the cathode resistor is bypassed with a capacitor, the voltage across the resistor remains constant which keeps the bias voltage constant.