The difference between the fractions $\frac{9}{10}$ and $\frac{1}{5}$ is $\frac{7}{10}$.

To determine the difference between two fractions, you need to subtract one fraction from the other. The first step is to ensure that both fractions have a common denominator. The denominators of $\frac{9}{10}$ and $\frac{1}{5}$ are $10$ and $5$, respectively. The least common multiple (LCM) of $10$ and $5$ is $10$.

Next, convert $\frac{1}{5}$ into a fraction with a denominator of $10$. To achieve this, multiply both the numerator and the denominator of $\frac{1}{5}$ by $2$, resulting in $\frac{2}{10}$. Now, you have the fractions $\frac{9}{10}$ and $\frac{2}{10}$.

Now you can subtract the smaller fraction from the larger fraction:

Thus, the difference between $\frac{9}{10}$ and $\frac{1}{5}$ is $\frac{7}{10}$. This method ensures that you accurately compare and subtract fractions by using a common denominator, making the process clear and straightforward.