How do coordinates work in minecraft? (SSP)

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.