What exactly is the difference between gingivitis and periodontitis?

Big difference: Gingivitis refers to inflammation of the gums due to an excess of plaque on the teeth. Signs of gingivitis include red, swollen gums, or gums that bleed easily when you brush your teeth. If left untreated, this will lead to periodontitis which is a more serious gum disease. Some signs of periodontitis include loose\ drifting teeth, gum recession, perio pockets and bone loss, infections & pain.

Attachment loss: The difference is attachment/bone loss. Gingivitis is a reversible inflammation of only the gum tissue. It becomes red and irritated due to microbial plaque. Periodontitis is the progression of gingivitis, where the microbial plaque and bacteria move deeper beneath the gum line and destroy the connective tissue, ligament's, and bone that support your teeth!

Similar: Gingivitis is an inflammation of your tissues and usually reversible with cleaning or debridement. Periodontitis is when your gums and the bone surrounding the teeth are compromised or have bacteria causing periodontal disease. This is not cleaned or treated with simple cleanings and check-up and may necessitate further treatment than rountine cleaning.