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They are similar in that they are both open source browser extensions that block advertising. However, the names are quite confusing. Adblock Plus is actually a fork of the original Adblock application. Adblock is not related to the original Adblock or Adblock Plus. So, to put it in somewhat confusing terms, Adblock Plus is a version of Adblock that is not related to Adblock. Also, Adblock was developed for Chrome and Safari when Adblock Plus was only available for Firefox.
Adblock supports Adblock Plus filter subscriptions, so functionally there is really very little difference between the two. There may be some difference in blocking video ads, but to be honest I’ve been using Adblock for quite a while with Chrome, and haven’t recently compared the two.
Also, one feature that I suggest you use is the ability to turn off Adblock/Adblock Plus on websites that you like and visit often. After all, that's the way the bills get paid, and it's only fair to support the websites that you frequent. Plus it helps ensure that they will continue to be there for you in the future.