It's a galaxy. It's M31, our neighbor the Andromeda Galaxy
. The picture is a mosaic of over seven thousand exposures made by the Hubble Space Telescope. NASA says it contains over 100 million stars. If you click on the image to enlarge it, you can scroll around it to see a lot more, though you won't be able to count 100 million stars.
Credits: NASA, ESA, J. Dalcanton, B.F. Williams, and L.C. Johnson (University of Washington), the PHAT team, and R. Gendler