What is the difference in the electronic configuration of transition and inner-transition elements?

Hint: The transition elements i.e. the d-block elements involve the filling of (n-1) d subshell whereas the inner- transition elements i.e. the f-block elements involve the filling of (n-2) f subshell. Here, n=1.2,3-----and so on.

Complete step by step answer:
The d-block elements are called the transition elements because they represent the change i.e. the transition in the properties of elements from highly electropositive metals of s-block elements to highly electronegative non-metals of the p-block elements. The elements of the d-block elements involve the filling of d-orbitals i.e. in which the last electron enters the last but one d subshell i.e.(n-1) d-subshell of the atom. Their general electronic configuration is: $\text{(n-1)} {{\text{d}} ^ {1-10}} \text{n}{{\text{s}} ^ {1-2}} $, where n=4 for the first transition series=5 for second transition series, n=6 for third transition series and so on.
The f-block elements are called as the inner transition elements because they are occurring in between the transition elements ie. the d-block elements. The elements of the f-block elements involve the filling of f-orbitals i.e. in which the last electron enters the last but two i.e.(n-2) f-subshell of the atom. Their general electronic configuration is: $\text{(n-2)} {{\text{f}} ^ {1-14}} \text{(n-1)} {{\text{d}} ^ {0-1}} \text{n}{{\text{s}} ^ {2}} $.
f-block elements occur in two series: lanthanides and actinides. In lanthanides, the 4f subshell is progressively being filled. It consists of 14 elements from Ce (58) to Lu (71) after La (57) and their general electronic configuration is: $\text{4}{{\text{f}}^{1-14}}\text{5}{{\text{d}}^{0-1}}\text{6}{{\text{s}}^{2}}$.

On the other hand, in cases of actinides, the 5f subshell is progressively being filled. It also consists of 14 elements from Th (90) to Lr (103) after Ac (89) and their general electronic configuration is: $\text{5}{{\text{f}}^{1-14}}\text{6}{{\text{d}}^{0-1}}\text{7}{{\text{s}}^{2}}$.

So, the difference in the electronic configuration of transition and inner transition elements is that,
In the transition metal in which the last electron enters the last but one d subshell i.e.(n-1) d-subshell of the atom and, thus, the electronic configuration is: $\text{(n-1)}{{\text{d}}^{1-10}}\text{n}{{\text{s}}^{1-2}}$.

On the other hand, in the inner- transition elements, the last electron enters the last but two i.e.(n-2) f-subshell of the atom and thus, the electronic configuration is: $\text{(n-2)}{{\text{f}}^{1-14}}\text{(n-1)}{{\text{d}}^{0-1}}\text{n}{{\text{s}}^{2}}$.

Note: Always remember that the transition metals start from the fourth period i.e.=4 up to the last seventh period i.e. n=7 whereas the inner transition metals start from the sixth period i.e.=6 and ends in the seventh period i.e. n=7.