How do coordinates work in minecraft? (SSP)

I can't figure out how minecraft coordinates work and they tell specific points in a world.

The coordinate system used in Minecraft is a little confusing, as it doesn't really correlate to what is generally used. Usually, the `X` and `Y` axes are used when talking about length and width, and the `Z` is used for height. However, in Minecraft, the `X` and `Z` axes represent length and width, and `Y` represents height.
As `X` or `Z` increase or decease, you are moving farther away from `0, 0`, or the "center" of your Minecraft world. Traveling south-east will increase your `X` and `Z` coordinates, and traveling north-west will decrease your `X` and `Z` coordinates. On the `Y` axis, 0 is the very bottom of the map, and 256 is the top of the map (or 128 in the Nether). Sea level is at 64 on the `Y` axis.
If, for example, your coordinates read `X: 300, Y: 65, Z: -200`, you would be 300 blocks east and 200 blocks north of your `0, 0` block, and at an altitude of 65 blocks.
• When you hit `F3`. See this question. – Kevin Yap Jul 3 '11 at 4:15