Decimals can be rational or irrational. To see why let's look at two examples. First, let's look at the decimal 94.340192. Notice that this decimal is both non-terminating and non-repeating. Rational numbers only includes decimals that are only terminating or repeating. Therefore, this decimal is irrational.
Let's look t another example. In this example, let's look at the decimal 5.252525... and see if it's rational or irrational. Since the decimal does not terminate, it could be an irrational number. However, notice that the 25 after the decimal point repeats and repeating decimals are rational numbers. Therefore, 5.252525... must be rational.
To answer your original question, not all decimals are rational. Always look to see if the decimal in non-terminating or non-repeating and you will get your answer.