The 'd sound' /d/ is voiced (the vocal cords vibrate while producing it), and is the counterpart to the unvoiced 't sound' /t/.
To create the /d/, air is briefly prevented from leaving the vocal tract when the tip of the tongue presses against the tooth ridge while the sides of the tongue press against the upper side teeth. The sound is aspirated when the air is released. The aspiration for a /d/ is less than the aspiration for a /t/.
Advanced ESL/ELL pronunciation students should be aware that when the /d/ is followed by an 'l sound' /l/ or an 'n sound' /n/, the aspiration of the /d/ is not released in the same manner as before other sounds. Instead, a lateral aspiration is used in the /d+l/ combination, and a nasal aspiration is used in the /d+n/ combination.