Obstruents are sounds that disturb the airway by completely stopping it or narrowing it to the point that it makes a hissing sound.  Examples of sounds that stop the air are /b/, /p/, /t/, /d/, /k/ and /g/.  These sounds are called stops. Example of sounds that restrict the airflow are /f/, /v/,  /s/, /z/, and ‘sh’ (the symbol for this sound is /ʃ/).