Out of the three possible circuit conf‌igurations for a transistor, common-emitter arrangement is more common in switching applications. So, henceforth, npn transistors will only be considered. -

Input characteristics : A graph between base current $I_{B}$ and base-emitter voltage $V_{B E}$ gives input characteristics. As the base-emitter junction of a transistor is like a diode, $I_{B}$ versus $V_{B E}$ graph resembles a diode curve. When collector-emitter voltage $V_{C E 2}$ is more than $V_{C E 1}$ , base current decreases as shown in Fig.1(b).

Output characteristics : A graph between collector current $I_{c}$ and collector-emitter voltage $V_{C E}$ gives output characteristics of a transistor. For zero base current, i.e. for $I_{B}=0$ ,as $V_{C E}$ is increased, a small leakage (collector) current exists as shown in Fig.1(c). As the base current is increased from $I_{B}=0$ to $I_{B 1,} I_{B 2}$ etc, collector current also rises as shown in Fig.1(c).

Fig.2(a) shows two of the output characteristic curves, 1 for $I_{B}=0$ and 2 for $I_{B} \neq 0$ . The initial part of curve 2 , characterized by low $V_{C E,}$ is called the saturation region. In this region, the transistor acts like a switch. The flat part of curve 2 indicated by increasing $V_{C E}$ and almost constant $I_{C}$ is the active region. In this region, transistor acts like an amplifier. Almost vertically rising curve is the breakdown region which must be avoided at all costs.

For load resistor $R_{C},$ Fig.1(a) the collector current $I_{C}$ is given by

$$I_{C}=\frac{V_{C C}-V_{C E}}{R_{C}}$$

This is equation of load line. It is shown as line AB in above figure. A load line is the locus of all possible operating points. Ideally, when transistor is on, $V_{C E}$ is zero and $I_{C}=V_{C C} / R_{C} .$ This collector current is shown by point A on the vertical axis. When the transistor is off, or in the cut-off region, $V_{c c}$ appears across collector-emitter terminals and there is no collector current. This value is indicated by point B on the horizontal axis. For the resistive load, the line joining points A and B is the load line.