Rohitchampion, a lot of basic questions. Detailed answers you certainly will find in relevant textbooks.
Some short explanations:
1.) The BJT needs a certain voltage Vce to work in the active region. Here, the BJT acts as a (not ideal) current source. That means: The output current depends somehow on the voltage Vce.
Because the gain determining resistor Rc causes a change in Vce (during amplification) the output resistance of the BJT alone is Delta(Vce)/Delta(Ic). This is a rather large value in the range of (20..100)kohms.
This value can be found directly by evaluating the SLOPE of the output characteristic Ic=f(Vce).
2.) More than that, this output characteristic (as a set of curves for various base currents) is a very illustrative and versatile tool to find a suitable bias point.
3.) The input characteristic Ib=f(Vbe) is a graphical illustration of the bias point dependent input resistance rbe. This resistance is important for calculating the allowable load for the driving signal source.
4.) There is another characteristic, which is very important: The transfer curve Ic=f(Vbe).
This curve shows how the transistor can amplify. It demonstrates how the output current changes with the input voltage.
As the most important transistor parameter, the SLOPE of this curve gives the transconductance g=Delta(Ic)/Delta(Vbe).
Hope I could help.