Friday Jan. 27 the iPad turns two years old. There were tablet computers before the iPad, just like there were smartphones before the iPhone. But just like the iPhone created the market for smartphones, the iPad created the market for tablets. And now it owns it.

Estimates put the iPad’s market share of tablets around 75%. Apple Inc. has sold millions upon millions of the handy mobile devices in just 24 months. Apple hasn’t released a total, but it did just release numbers for its fiscal Q1 2012, ended Dec. 31, 2011. The company sold 15.43 million iPads during the quarter.

Last week Apple debuted iBooks 2 for iPad, a store selling interactive, multimedia textbooks to students. One analyst estimates Apple sold 350,000 iBooks textbooks in the first three days. Through the iPad, Apple has an opportunity to corner the market on these cutting-edge tomes.

As of this month, 19% of U.S. adults own a tablet PC, most likely an iPad, according to a new study by the Pew Internet & American Life Project. That’s up from 5% in November 2010.

Tablets will reshape the e-retailing landscape in the years to come. Some savvy retailers already are optimizing their e-commerce sites for viewing on iPads and creating dazzling iPad apps. These retailers include Backcountry.com, Overstock.com Inc. and Wine.com Inc., all of whom happen to be speaking on tablet apps at the Mobile Commerce Workshop at the 8th Annual Internet Retailer Conference & Exhibition in Chicago June 5-8.

It’s only taken two years for the iPad to change the way people browse and shop the web. With Amazon.com Inc. selling millions of its new Kindle Fire tablet and iPad sales growing madly, imagine what tablets will do to e-retailing in the years ahead.