Every airline will use different boarding procedures, taking into account everything from priority passengers (business class, frequent flyers), what people have paid for, through to the quickest way to board the plane.
As a result, no two airlines will generally have the same process. To give two examples, American Airlines (which it looks like you were on) boards First Class, Business class, high-level frequent flyers, lower-level frequent flyers, then Groups 1 through 4.
Group 1 is people that have paid for early board, credit card holders, etc
Group 2 is people that use Online check-in, or are connecting from another airline
Group 3 is people that used a self-serve Kiosk to check-in.
Group 4 is people that used an agent to check-in.
By contrast, United Airlines boards using groups 1-5, which are :
Group 1 is Business/First class passengers, as well as high-level frequent flyers.
Group 2 is lower less frequent flyers, people that have paid for early boarding, and people with United-branded credit cards that allow early boarding.
Group 3 is all remaining passengers that are in window seats.
Group 4 is all remaining passengers in middle seats.
Group 5 is everyone else (ie, aisle seats)
(Both also have things like people with disabilities/wheelchairs/etc, military, and so on that I've ignored)