You get a queue of objects and when you click the screen, the next object appears. The first thing on every level is some sort of a floating platform - it may be a long plank, or a bunch of logs loosely bunched together (these can float apart, as you can see in the screenshot).
Once the first floating object is in the water, the next click usually drops a cow on it, and it happily floats. Of course, saving just one cow isn't enough. Cows are social creatures, so you have to save the whole herd. When you drop the next cow on the floating plank, it changes the balance and causes the plank to sink deeper into the water. If you're not careful, the whole plank might tip over -- and there go the cows! You may receive extra planks later on, which you can use to bolster up your platform.
There are bulls, too, and a bull weighs four times as much as a cow -- so you need to be extra-careful with them. Happily, though, you don't have to save every cow. I mean, every cow is precious, but you can usually get away with saving only nine out of twelve or so and still pass the level.
Freaky Cows is highly recommended, and I greatly enjoyed the frantic mooing in the soundtrack.