What's the difference between salt and fresh water?

Spot on, Alan. They are, as you say, the same chemical. H2O is water.

The water cycle on Earth is why we have fresh water.

What happens is the energy from the Sun hits the Earth's surface and the oceans' surface. Every square metre of the Earth gets about one kilowatt of energy coming from the Sun.

That's like having a one bar electric fire shining on it. That heat evaporates the water from the surface of the ocean.

Water can leave quite easily, but the heavier elements and ions which are dissolved in the water find it much more difficult, so they stay behind.

What evaporates is essentially fresh water. This goes up and forms clouds. These then drift across the ocean's surface until they're forced to rise over, say, a mountain range or something.

As the clouds are forced to rise they find it much harder to hold on to water which is condensing inside them, clouds are just massive bodies of tiny droplets of water.

Water rains down on Earth and falls on the ground as fresh water. As it percolates through rocks and rivers it absorbs small amounts of salts on the way through becoming slightly salty in the process but not perceptibly salty.

As it slowly drains into the ocean it carries those salts with it to the sea, which picks up those salts.

What leaves the sea is fresh water again, so the sea is continuously becoming slightly more salty.

Why is the sea not becoming more and more salty? The answer is it's become about as salty as it's ever going to because it's now reached sort of equilibrium position where if you add more salt to the sea chemical reactions kick in and take it out again.