Sticky's are just topics/threads that stick to the top of the list - instead of getting replaced with newer posts. It keeps important topics where people can easily find them.
If you have a Sigil related question, how to use the product etc., this is the place.
If it is a general ePub related question then you can use the ePub forum...there are SOME smart people that can answer questions on ePubs over there...but not too many...if they were really smart they would be using Sigil!!
Workshop seems to be a generalized forum. Writer is for author related issues. Apple devices if you have a question for them...etc.
There is a short description of each forum under the title of the forum itself that should help guide you around.
Cheers!
[doh...out typed by Diap!]