The Q-point or quiescent point of a transistor is considered during amplification, as this is the point of amplification where the signal is completely amplified without any kind of attenuation. This is located when the transistor is operated in the active region.
When a value for the maximum possible collector current is considered, that point will be present on the Y-axis, which is nothing but the Saturation point. As well, when a value for the maximum possible collector-emitter voltage is considered, that point will be present on the X-axis, which is the Cutoff point.
When a line is drawn joining these two points, such a line can be called as Load line. This is called so as it symbolizes the output at the load. This line, when drawn over the output characteristic curve, makes contact at a point called as an Operating point or quiescent point or simply Q-point.
The load line is drawn by joining the saturation and cut off points. The region that lies between these two is the linear region. A transistor acts as a good amplifier in this linear region, which is a part of the active region. The active region of a transistor is one in which emitter-base junction is forward biased and the collector-base junction is reverse biased.
The operating point should remain stable to achieve Faithful amplification. If this load line is drawn only when DC bias is given to the transistor, but no input signal is applied, then such a load line is called as DC load line. Whereas the load line drawn under the conditions when an input signal along with the DC voltages are applied, such a line is called as an AC load line.