Ground chuck is ground beef, but not all ground beef is ground chuck. This is because "ground" refers to the way the beef is package, and "chuck" refers to which part of the cow it was taken.
The chuck cut of beef is taken from the upper part of the cow around the neck. Other types of ground beef can come from the round (or butt) or the sirloin (or back).
Retailers sometimes relate the cut of meat to the percentage of fat in the meat, though this is not always accurate. The leanness of the meat is not always determined by the part of the cow from which it is cut.