This is the big one and arguably creates the most confusion and can cause some unfortunate owners of wearable devices to find themselves staring down at a broken piece of kit. That's because these are two very different things and there's no clear consensus on the definition of the two terms. Water resistant is regarded as the lowest level of protection and essentially means that something is designed to make it difficult for water to infiltrate.
Waterproof should mean that something is impervious to water. The problem is that there's no established industry standard to classify something is waterproof. That's why you tend to see some things labelled waterproof, but it actually isn't. That's why things get tricky.