The Penn World Table was developed by Robert Summers and Alan Heston (and others) to facilitate consistent national accounts comparisons across countries as well as over time. As noted by Summers and Heston:
The standard national accounts archives of the various international organizations, following the United Nations "System of National Accounts" (SNA), allow only intertemporal comparisons within countries. The Penn World Table is an attempt to get closer to a System of RealNational Accounts (SRNA) that makes possible interspatial comparisons as well. ("The Penn World Table (Mark 5): An Expanded Set of International Comparisons, 1950-1988", Quarterly Journal of Economics, May 1991, p327, http://www.jstor.org/pss/2937941)
Thus the national accounts time series of the Penn World Table have the unique feature that the expenditure series are denominated in a common set of weighted average international prices (IP) in a common currency (the US dollar) so that real quantity comparisons can be made both among countries and over time.
A substantially revised version of the database was released in July 2013. As noted by Feenstra, Inklaar and Timmer:
Effective with version 8, the Penn World Table (PWT) will be taken over by the University of California, Davis and the University of Groningen, with continued input from Alan Heston at the University of Pennsylvania. Version 8 will expand on previous versions of PWT in three respects. First, it will distinguish real GDP on the expenditure side from GDP on the output side, which differ by the terms of trade faced by countries. Second, it will distinguish growth rates of GDP based on national accounts data from growth rates that are benchmarked in multiple years to cross-country price data. Third, data on capital stocks will be reintroduced. ("The Next Generation of the Penn World Table", April 2013, http://www.rug.nl/research/ggdc/data/pwt/v80/the_next_generation_of_the_penn_world_table.pdf)
The latest version 8.0 of the Penn World Table contains 29 variables for 167 countries (with two versions for China). The approximately 6,000 annual time series begin as early as 1950 and end generally in 2011.
The Penn World Table is updated at irregular intervals. PWT 5.5 was released in 1993, PWT 5.6 in 1995, PWT 6.1 in 2003, PWT 6.2 in 2006, PWT 6.3 in 2009, PWT 7.0 in 2011, PWT 7.1 in 2012 and PWT 8.0 in 2013.
In dX format the Penn World Table is organized "By Variable" where each table contains a single series for all countries and "By Country" where tables show all series for a single country. Also for reference purposes in the dX format Penn World Table database, previous versions of the PWT beginning with 5.5 are included "By Variable".