Are there any synchronization primitives like Barriers, Semaphors, Locks, Monitors, ... available in JavaScript / Web Workers or is there some library available empowering me to make use of such things (I'm thinking of something like java.util.concurrent in Java)?
Do Workers have obscure properties which differentiate them from Threads (can they share memory with the main thread, for example)? Is there some kind of limit how many workers can be spawned (like, for security reasons or something...)? Do I have to take special care of something?
Web workers don't have a concept of shared memory; all messages that are passed between threads are copied. With that being said, you don't have Barriers, Semaphores, Locks, and Monitors, because you don't need them in the web worker model.
The concept of shared memory was proposed back in Feb 2011 but the status is now wontfix due to developer complexity => https://lists.webkit.org/pipermail/webkit-unassigned/2011-February/287595.html
There is also a nice blurb about web workers here. http://blogs.msdn.com/b/ie/archive/2011/07/01/web-workers-in-ie10-background-javascript-makes-web-apps-faster.aspx
Hope this helps