A Mining Turtle is the same as a Turtle, only now you can use it to do your mining for you. A great place to get started with this is right click on the Mining Turtle once placed to open the GUI. Type in excavate then a number (e.g excavate 3). By typing excavate 3, you are telling it to mine 3x3 hole down to bedrock. It can return before hand if it encounters a problem. By typing excavate 9, you are telling it to mine a 9x9 hole to bedrock.
So you might be thinking, ok, excavate 100. Well, unfortunately, it isn't that straight forward: it only has 9 inventory slots and once they are filled, the Turtle will return to the place where it started mining from. To continue mining the same size hole, break the turtle and return it to the next possible starting point that has the same y coordinates as the original starting point.
If you put a chest adjacent to where the mining turtle starts, the turtle will empty its contents into the chest. It will also empty the coal into the chest and so may run out of fuel.
Since it mines every minable block at the same speed, the mining turtle is also very useful for quickly mining obsidian.