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General question about jobservers and runtimes ...

Hi all,
today I've found that I have a general problem in my understanding of jobserver and runtimes.
After having read the first documentations from Talend I had the idea that the jobservers are some kind of execution environements or containers that may be in a specialized form Talend runtimes i.e. Karaf engines.
What I did (and do) not understand is what is a Jobserver if it is not a runtime / karaf? Is it simply a filesystem environment where the jobs are executed as standalone executables or how does it work?
The second thing I'm wondering about is that there seem to be two karaf containers in a default installation one under /opt/Talend-5.2.2/esb/container/ and the other under /opt/Talend-5.2.2/runtime/.
What is the purpose of the two and which is the right one for the execution server in the TAC?
It would be nice if you could explain a bit more detailed. The documentation of Talend is wide + okay but in the details I miss sometimes some more in depth explanation.
Many thanks