Delete user from Windchill does just that, deletes the user ID entries in the Windchill database. Remove Participant, only removes the user entry from the Participants page. No Windchill or WindchillDS action is performed.
I am guessing you are on Windchill10.
The options on the Participants screen are: Add Participant - Add users to the screen Remove Participant - Remove users from the screen New User New Group New Organization Remove from Cache Delete from Windchill - Deletes the user from the Windchill database which removes them from Lifecycles, workflows and teams, entry is still in WindchillDS and the user can still log into Windchill Delete from Windchill and Windchill Directory - Deletes the user from Windchill and WindchillDS, cannot log into Windchill Search Disconnected Participants Purge Participant Cache