In normal speech, when vocal organs which have held back the flow of air are released for such sounds as such as /p/, /b/ (the lips), /t/, /d/ (the tongue and alveolar ridge), /k/ or/g/ (the tongue and velum) before a vowel there is slight aspiration.
If these consonants are not followed by a vowel, they vocal organs may not be released at all. We then have a stop.
Aspirated (plosive): Stop at the red light.
Possibly not aspirated: Stop!
Not aspirated (stop): Stop the car.