The ggplot2 package is a data visualization package for R, which implements the so-called grammar of graphics, and makes it easy and accessible.
Hadley Wickham mainly created the ggplot2 package back in 2005. Since then, it has grown to be one of the most popular packages of the R language, and a huge community formed around it. Its main focus lies in making plotting in R very accessible, and so, it can be used as a replacement for the base graphics system.
The package, basically, is a successor of the famous lattice package and tries to take from it only the good parts and leave out the bad. So, it wants to make the creation of graphics easier.
On February 25, 2014, Hadley Wickham formally announced that ggplot2 is shifting to maintenance mode. This means that features will no longer be added, but major bugs will still be fixed. This is not because they lost interest in the package, but because it is feature-complete, meaning that the package includes all the features needed to create powerful graphics and adapt them to your needs.
The Grammar of Graphics is a book written by Leland Wilkinson in 2005. It is a framework that tries to sum up elements of designing, implementing, reading, and understanding graphics. Wilkinson divides the process of graphics creation into these steps: data transformation, scale, coordinates, elements, guides, and finally, display. The first part in this process, described as data, includes the actual statistical computations done on the given dataset that we want to visualize. Hadley Wickham tried to reflect these steps with the ggplot2 package and managed it very successful.