I think you must mean solving recurrences of the form T(n) = aT(n/b) + f(n).
From the relations that you have written I understand that you must be trying to learn the Master Method for solving recurrences. This is used where a divide and conquer strategy is used by the algorithm to solve problems. The D&C strategy divides a given problem into specific parts in order to solve it effectively.
Here a implies the number of parts a single problem is divided into.
(n/b) is the size of each sub-problem and
f(n) is the cost of solving(combining and splitting) each sub problem.
More details of the master method can be understood from here as suggested earlier by Raphael