What is the difference between excitation table and truth table?
Truth table shows the corresponding outputs for every combination of inputs. 
Consider the truth table of jk flip flop below

Q(t+1) is new output
Q(t) is present output
Q(t) bar is compliment of present o/p

Now look at excitation table below


Excitation table shows what should be the values of j and k so as to change the present output to the desired value (next output)

Recall 
Q(t+1) is new output
Q(t) is present output

1st combination: present state is 0 and we have to maintain same value to be next state meaning Q(t) and Q(t+1) is 0. Look at truth table. We can "reset" the o/p to 0 or maintain "unchanged" state. That is j and k should have either 1st or 2nd combinations of truth table. (J,k) is (0,0) or (0,1). Means j "must" be 0 and k is dont care (maybe 0 or 1).

2nd combination: present state is 0 and we have to change it to 1. Meaning Q(t) is 0 and Q(t+1) is 1. Look at truth table. We can "set" the o/p to 1 or "toggle" the state from 0 to 1. That is j and k should have either 3rd or 4th combinations of truth table. (J,k) is (1,0) or (1,1). Means j "must" be 1 and k is dont care (maybe 0 or 1).

3rd combination: present state is 1 and we have to change it to 0. Meaning Q(t) is 1 and Q(t+1) is 0. Look at truth table. We can "reset" the o/p to 0 or "toggle" the state from 1 to 0. That is j and k should have either 2nd or 4th combinations of truth table. (J,k) is (0,1) or (1,1). Means j is dont care (maybe 0 or 1) and k "must" be 1.

4th combination: present state is 1 and we have to maintain same value to be next state. Meaning Q(t) and Q(t+1) is 1. Look at truth table. We can "set" the o/p to 1 or maintain "unchanged" state. That is j and k should have either 1st or 3rd combinations of truth table. (J,k) is (0,0) or (1,0). Means j is dont care (maybe 0 or 1) and k "must" be 0.

Hence excitatation table is reverse of the truth table and is applicable only in seqential ciruits where the system remembers what the earlier output is.