On the Chronas homepage there are eleven images that are representative of the world at different times. For example, there is an image of a painting of Genghis Khan and the title is 1248: Mongols Invade East-Europe. Click on the image and you can read a short article about Genghis Khan and his empire. Click the map to the right of the article and you'll be taken to an interactive map of the world as borders appeared in 1248.
Once you are on the interactive Chronas map you can adjust the time slider at the bottom of the page to see national boundaries change through the course of history. Stop the time slider at any point and click on the map to reveal a Wikipedia entry about that nation. One of the neat aspects of the map is that as you get into the history of the United States, the Chronas map continues to show the sovereignty of Native American tribes. A lot of other interactive maps of national boundaries don't do that.
Somewhat hidden in the Chronas map is an option to explore various sets of data. This option is found in the upper, left corner of the map. In the data sets you can find things like "sunburst" visualizations of population demographics according to year. The sunburst allows you to start with a a broad data selection and refine it as you drag your cursor to the edges of visualization. For example, I chose the year 1959 then selected Hindu then dragged my cursor to see the geographical distribution of people of the Hindu faith. You'll also find aggregations of data that show you population distribution by ruler/ empire.
The final piece of Chronas to note is the option to turn on additional markers for cities, battles, artifacts, and famous people. Activate the additional markers and they'll appear on map in the proper geographic context for the time you have selected on the map's time slider. Each marker is interactive. Click the marker to be taken to a Wikipedia entry related to the item represented by the map marker.