Yes - you have editing ability for a short time afterwards, but most forums disable the edit ability after a while so that people can't go back and remove stuff they said and make a complete nonsense of a thread. Not sure how long it is on here - from 20 minutes to an hour is fairly common, as it gives the ability to tweak things after posting but not to change the content later.
Moderators and admins will be able to edit at any time so if it's critical you could ask one of them nicely