To view the history of changes to the task description, click on the History button in the description area. Use the scroll bar on the bottom to view the changes across time and users who made them. Text highlighted in green were added, while those highlighted in red were removed.
Users will need view permissions in tasks to perform this action. Depending on the user's project permissions, they may be able to view all the project tasks or just a subset of them. Watchers specifically added to the task will be able to view it.