If unchecked, the decay works nibbles its way inside the tooth, into the soft dentine. This is the bit when you might start to feel some pain. The rot speeds up, and the softer stuff inside your tooth can't support the harder outside. So the tooth can break, or cavities appear. And if you get an 'abscess', bacteria can spread and cause infections in other teeth and even elsewhere in your body.
Erosion is different from decay, but can cause just as many problems.
It doesn't involve bacteria but is caused by eating and drinking things that have lots of acid in them. Things like citrus fruits, pickles, fruit juice and fizzy drinks. And like the early stages of decay, you probably won't even notice it's happening.