Who leaves Hell?

Here's something that has been on my mind lately... We all understand that in Islam, once humanity is judged, some will go to heaven and some will go to hell. Heaven is eternal, and as I understand, so is Hell (for some people anyway...).

But I want to know who exactly Hell is eternal for... I've heard many times that some people will pay for their sins in Hell, and then be admitted into heaven... Some people say this only applies to sinful Muslims, but I want to know if Non-Muslims can come under this category as well. Can someone who isn't Muslim eventually leave Hell? Any clarification would be greatly appreciated.