Structured markup is code placed on a web page that helps search engines understand the content more accurately. You can use it, for example, to clarify if you are showing a list of products, list of reviews, list or recipes, etc.
Structured markup is also referred to as Rich Snippets or Schema markup.
Structured markup can have a big effect on how your pages show in Google's search results. Used correctly they can display a picture of you, breadcrumbs, the price and availability of a product (or list or products) and a star rating from reviews.
Obviously making your entry in the search results clearer and more attractive helps improve your Click Through Rate and this in turn is a factor in helping your rankings.
But some webmasters and spammers went overboard with this. A favourite technique was to claim, in the code, that there was a list of reviews and they were all five stars. However when the visitor clicked through they found no reviews.
In February 2014, to combat this, Google started banning websites that were obviously abusing their structured markup code.
This is a penalty, you will need to take action and then submit a reconsideration request.
In general you are going to have to ask Google to reconsider your website and convince them of two things:
Before you start any of the following use a notepad or document to keep a note of each thing you do.
Remove all structured markup code where the markup entity is not visible to the visitor. So, for example, if you say in the markup your is a list of 23 products between $10 and $30 say this visibly to your visitors as well.
If you say you have 24 reviews with an average rating of 4.8 stars, show those 24 reviews on the same page.
Remove Google authorship from places such as home pages where there is no actual content.
Basically, if you're saying it to the search engines, say it to the visitors. Otherwise it is no more or less spammy than hidden text (text placed on a background of the same colour to try and add certain keywords to a page).
Now write up all the actions you have taken for each page or set of pages as part of your reconsideration report. Right now (as of early 2014) there has been a fair amount of confusion regarding rich snippets by have-a-go webmasters so you will probably be able to get away with a "fair cop, my mistake, sorry, I now know better" approach.
If the work was carried out by a SEO company that you hired and it was aggressively spammy drop the SEO company, document it and include this in your reconsideration request.
I'm Tim Hill, a Search Engine Optimization and Online Marketing specialist. I created this site to help others understand that SEO is not a mysterious black art!.
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