What is the difference between good and bad bacteria?

Not so fast.... Bad bacteria have characteristics that can cause severe illness and death. However, it's not all black and white... What can be good bacteria in certain settings in our body (for example, e coli, is a bacteria that is found in our colon), can be considered "bad" bacteria - since it is found in a body site that should be otherwise sterile (eg. E coli from urine in a pt with burning on urination).
Help v disease. "good bacteria" are companions on our journey through life, providing necessary elements for our physiology that we can't make for ourselves. Things like vitamins, digestion aids, intestinal balance. "bad bacteria" are similarly constructed microorganisms that cause disease. Some bacteria can be both beneficial and harmful. Go figure, evolution is amazing.
See below. There are many pathogenic bacteria that cause disease in humans including staphylococcus aureus, streptococcus pyogenes and streptococcus pneumoniae. These could be considered "bad bacteria". Probiotic bacteria are those that promote health benefits. These "good" or probiotic bacteria are found in yogert, kefir and lassi. They include various lactobacilli, bifidobacteria among others.