Jquery & java script cross browser implement

Jquery & java script cross browser implement

I read about jquery and java-script and I was asking my self the next question:
What is it about jquery that turns it to be a cross browser language while java-script is not?
 
Can someone please explain to me what turns Jquery to be cross browser language and why java script isn't?
 
It means that every code that i write with jquery should be supported in all browsers? including internet explorer 6-7-8?
 
Include all Jquery plugins? 
Please explain.