The former standard was 4 x 3, and most Powerpoints are still created using this aspect ratio.
The screenshots below illustrate the difference between the two. This is the same aspect ratio that you see on most new computer screens. Most screens these days, including those in the large lecture rooms at the Medical Sciences Building downtown and the Terrence Donnelly Health Sciences Centre in Mississauga, have screens with an aspect ratio of 16 x 9 or 16 x 10.