Adware displays advertisements, usually in a web browser.
Junkware simply refers to software from not well known companies) that the user could have easily avoided installing but didn't pay attention and flew through an installer from another program, whether or not this piece of junkware.
I don't think that this necessarily includes software that is actually junk. For example, Norton and McAfee products are junk, but people want and use them because they believe that they work (except for the security scanner someone might get from Adobe
Flash Player). Another example is that the Google toolbar, and Chrome-which we know is very widely used and loved-comes bundled with Flash Player, but because users don't realize there's a checkbox to decline it, they ask for help removing it because it's