Interphase is the period of the cell cycle where the cell is actually growing, developing, and carrying out cellular processes. Essentially, interphase is when the cell is busy being a cell. It grows, synthesizes proteins, replicates its DNA, and so on and so forth.
Mitosis is only the small portion of the cell cycle when it is dividing into two new cells. The rest of the time (interphase), the cell is performing its various functions.
