There are many types of bacteria that can make up the plaque, but we can simply separate them into gum-friendly and not friendly. Gum-friendly plaque is aerobic, it uses oxygen to survive and its by-products do not harm gums as much. If the plaque is left undisturbed around your gums, it will change to include more unfriendly anaerobic bacteria. These do not need oxygen to survive, but your gums do not like their by-products at all.
Gums faced with anaerobic bacteria try to fight against them by filling with more blood, swelling and separating away from the teeth. Consequently, they bleed when brushed or flossed. If your gums bleed when you do either of these, you may have gum disease. Gum disease is bad and can cause the loss of teeth if left unattended - but it is treatable.