Chrome supports two enormously popular ad blockers, AdBlock and Adblock Plus, that work almost identically. Despite their similar names, Adblock Plus isn't an improved or paid version of AdBlock -- the two free extensions have different developer, and both provide the same basic function. Whichever you choose, click its icon on Chrome's menu bar to change its settings or allow ads on a particular site.
Chrome's most popular ad blocker, AdBlock, blocks image ads, text ads and in-video ads on sites across the Web. It downloads its filter lists automatically, with the option to import or create your own filters, and it can show ads only on specific websites. AdBlock also has toggles to show Google search ads or to allow ads on specific YouTube channels -- a feature not found in Adblock Plus.
Though not as popular in Chrome, Adblock Plus is the most popular extension of any type across all browsers. Most features in Adblock Plus work just like those in AdBlock: ABP uses automatically updated filter lists to block all types of ads and it lets you create your own filters or whitelisted websites. One unique option in AdBlock Plus allows through "non-intrusive" ads that follow criteria set by the developer, such as not using animation.
Origin of the Names
Though it seems backwards, AdBlock is actually inspired by Adblock Plus, which came out first as a Firefox extension. Adblock Plus, in turn, was an improvement on a now-defunct Firefox extension called "Adblock" -- note the lower case "B." AdBlock (capital "B") came out for Chrome prior to the Chrome version of Adblock Plus, and now both are available for free on the Chrome Web Store.