Pressing the F3 key will bring up the games diagnostic HUD.
There is a lot of technical (cryptic) information, but some key parts are the GPS and Mob identifiers.
On the left side, halfway down the page, there is a set of coordinates x, y, and z.
these coordinates are accurate out to 10 or 12 decimal places... all that is really needed is the integers making note of positive or negative.
The y coordinate is ALWAYS positive and is an indication of altitude. value 0 (zero) is the bottom of the world and the start of the VOID. Seldom will you ever get a value below 4 for two reasons. First, the coordinate value is of the center of your head, as you are about 1.8 blocks tall and the center of your head is 1.6 blocks from your feet, even standing on the VOID, your y > 1.6. Second, there is a rough layer of Bedrock, otherwise known as Adminium 1 to 4 layers thick. A side note, Adminium is indestructible in the normal coarse of game-play. Sea level is at about level 64 and level 128 is the highest block location.
The x coordinate is your position North and South, travel North will make x smaller, travel South will make x larger.
The z coordinate is your position East and West, travel East will make z smaller, travel South will make z larger.
The cardinal location x=0 z=0 is the initial spawn location and is were you start the in the world and is were you re-spawn when you die.
The HOME base in this world is located at x=41, y=65 z=-55
The f value is your facing and will range from 0 to 3. Facing West has an f value of 0. facing North has an f value of 1. Facing East has an f value of 2 and facing South has an f value of 3
The second benefit of the Diagnostic Mode is each and every mob will have their ID above their head and this number can be seen through all blocks. When in diagnostic mode, look into walls, floor and ceilings, step to the right and left and you may see these numbers, some will appear close, others far way. This is a very good way to find caves and tunnels.
Damn... the Mob locator has been disabled in 1.8
I had not realized how much I relied on this feature!