First of all, we need to define our terms. A lot of people that I talk to are confused about this subject, simply because some translations in the bible use these terms interchangeably. In fact though, they are not interchangeable and represent different types of creatures that were on the earth.
The Watchers – These are fallen angelic beings that left their first estate. Genesis 6 verifies this, but mostly they are talked about in the extra-biblical book of 1 Enoch. As with most extra-biblical texts, it needs to be interpreted as human, fallible, and ultimately nowhere near the same level as scripture. But, that doesn’t mean it doesn’t contain insight, and since we know that angels descended in Gensis 6, this is the only other info available. Anyhow they are pretty straight forward.
Nephilim – These are the first generation offspring of the watchers and humans. These giants were said to be 90 feet tall, and had super-human-like abilities. The Nephilim are likely the creatures written about in ancient greek mythology, known as the titans. While their names are different, the descriptions sound astonishingly similar.
Giants – These are the later generations of Nephilim. It seems that in the beginning, the first generation giants were so huge, they got the nickname “cloud eaters,” but later generations grew shorter and shorter until they settled at around 9 feed tall.