First, you have regular ground which commonly comes from the plate (underbelly) and breast of the cow. This is not the choicest of meats and tends to be very fatty, grisly and, in lesser grades, can contain some bone chips. Regular ground beef is the least purchased ground simply because Americans are becoming more health conscience and don't mind paying an extra 50 cents per pound for better quality. This area on the cow is also where corned beef and brisket come from. Usually, when your butcher or their packager is processing regular ground beef, they will add in fat found from other parts of the cow. This gives the percentage of fat figure that is seen on your ground beef packaging (e.g. 75-80% lean means that 20-25% of the package is really fat, also known as suet).
Secondly, you can purchase ground chuck. This is the most popular ground on the market. Ground chuck comes from the shoulder and neck area of the cow and is usually very fatty but also has an immense amount of flavoring. This is why such cuts of meat as a chuck roast are favored for a hearty winter meal. Served with potatoes and gravy, this cut can be especially delicious but has a downside of too many fat calories. Sometimes, when processing ground chuck, the butcher will blend a better grade of fat (if necessary) in with the meat. This fat comes from other "chuck type" cuts of meat such as rib-eye steaks. Ground chuck is great for a juicy burgers and mouth watering meatloaf.
Your third choice is ground round. The "round" area is actually the rear or rump of the cow. This is the area where a rump roast, round steaks and round roasts come from. This meat is high in flavor, very lean and also a tad bit chewy. Perfect for a slow cooked pot roast that won't accelerate your fat intake. Ground round is an ideal choice for tacos, lean meatloaf and sloppy joes. It also works well with most Hamburger Helpers since it requires little draining due to the lack of fat content.
Lastly, ground sirloin is approaching almost a cult-type status among the very health conscience who don't want to sacrifice on taste. Generally, a good sirloin cut of meat will have a marbling texture of fat throughout the meat that is completely different from the chunks found in a chuck. This gives an almost angelic blend of fat and flavor for burgers that will truly out of this world.