3 Answers

You can play it here for free, you just have to put up with commercials http://www.popcap.com/games/plants-vs-zombies/online or you can purchase it from the same page to get rid of the commercials. 

Plants vs. Zombies is a tower defense video game developed and originally published by PopCap Games for Microsoft Windows and OS X. The game involves a homeowner using many varieties of plants to repel an army of zombies from "eating their brains". It was first released on May 5, 2009, and made available on Steam on the same day.[1][10] A version for iOS was released in February 2010, and an HD version for the iPad.[11] An extended Xbox Live Arcade version introducing new gameplay modes and features was released on September 8, 2010.[4] PopCap released a Nintendo DS version on January 18, 2011 with content unique to the platform.[12] The PlayStation 3 version was released in February 2011 also with added new co op and versus modes found in the Xbox 360 version. An Android version of the game, exclusive to the Amazon Android App Store, was released on May 31, 2011.[13] On February 16, 2012, a version was released for BlackBerry PlayBook.[5] Later a BlackBerry smartphone version of the game was released on January 2013 following the launch of BlackBerry 10.[6] Furthermore, both the original Windows and Mac version of the game have been re-released with additional content in a Game of the Year version.
The game received a positive response from critics, and was nominated for multiple Interactive Achievement Awards, alongside receiving praise for its musical score. PopCap officially announced that a sequel called Plants vs. Zombies 2: It's About Time would be released in the summer of 2013, sometime after the initially announced July 18 launch date.

You can get the game play description from WIKIPEDIA, where I got the information provided, and this graphic:
 

Here's some ZOMBIES for you're game