The Mohs scale was devised by Friedrich Mohs in 1812 (and therefore it's never spelled "Moh's"). You use the scale by testing your unknown mineral against one of these standard minerals. Whichever one scratches the other is harder, and if both scratch each other they are the same hardness.
The Mohs scale is strictly a relative scale, but that's all that anyone needs. In terms of absolute hardness, diamond (hardness 10) actually is 4 times harder than corundum (hardness 9) and 6 times harder than topaz (hardness 8). Because it isn't made for that kind of precision, the Mohs scale uses half-numbers for in-between hardnesses. For instance, dolomite, which scratches calcite but not fluorite, has a Mohs hardness of 3½ or 3.5.
There are a few handy objects that also fit in the Mohs scale. A fingernail is 2½, a penny (actually, any current U.S. coin) is just under 3, a knife blade is 5½, glass is 5½, and a good steel file is 6½. Common sandpaper uses artificial corundum and is hardness 9; garnet paper is 7½.
Mohs hardness is just one aspect of identifying minerals. You also need to consider luster, cleavage, crystalline form, color, and rock type to zero in on an exact identification. See this step-by-step guide to mineral identification to learn more.