windows GetMessage function

[trogne]

I'm trying to understand this description of GetMessage windows function :

"Retrieves a message from the calling thread's message queue. The function dispatches incoming sent messages until a posted message is available for retrieval." https://msdn.microsoft.com/en-us/library/windows/desktop/ms644936(v=vs.85).aspx

It doesn't make sense to me. What's the difference between "incoming sent message" and "posted message available" ?

The function waits for a message, but waits only for a "post message" ?

And about "until" : it returns false when it happens ?

Does "dispatch" means the function doesn't care about the message, and waits until there's a "post message" ?