Announcement thread/topic
An announcement is a special post type that is always displayed at the top of a forum. The purpose of an announcement is to increase the visibility of certain topics. Note that not all Communities members can make a post an Announcement -- those who can will be able to do so from the "Prioritize at the top of forum lists" option list towards the bottom of the Advanced Reply Editor.
Sticky thread/topic
A sticky thread/topic is a special post that sorts to the top of a forum for a specified amount of time. A sticky thread is similar to an announcement, whereas an announcement is displayed separate from other thread/topics and usually does not allow replies. Note that not all Communities members can make a post "sticky" -- those who can will be able to do so from the "Prioritize at the top of forum lists" option list towards the bottom of the Advanced Reply Editor.
Locked thread/topic
A Locked thread/topic is a special post that does not allow replies. Once a user locks a post or an administrator/moderator locks a thread/topic, no more replies in that thread are allowed.