Adblock still has me confused?

[Steve Dogan]

Is there a difference between adblock & adblock plus? Is there a difference when it says "extensions"? What's that term mean?
Is it free or does it only come with a "free trial buy it after that"? I've been told it's free from adblockplus.org but that's not installing on my IE [I think I have IE 10 & windows 8 ] When I manage to download it, at the end of the process I get a message saying "trial has expired. Please register" At the bottom it says "purchase now" in a little box. And that's on any site.
I've also been told there's a difference between adblock & adblockplus & that 1 is a risky download. True or no?
So now I'm totally lost. Can anyone put all this into "adblock 101"?
It worked the first time for a month free then I got the "trial expired. Please purchase". The 2nd time it was harder to get to work but somehow I got another 30 day thing. Now no matter what site I go on it says trial expired. Other people told me it's free period & indefinitely.