Making up the majority of the Earth's crust, rock is usually defined as a mixture of common minerals. Rocks can be hard or soft, as small as a grain or as large as a building. They have been an integral part of the history of mankind, first being used as tools for hunting and defense, and as a building materials to construct shelters and monuments. Combined with the effects of tectonics, weathering and vegetation, rocks define the natural landscapes we see around us. The minerals and metals we find in rocks are essential to the prosperity and cultural splendor of human civilization.
There are many kinds of rock, and they can be classified in a number of ways. However, geologists classify rocks based on how the rocks were formed. The three classes are igneous rocks (formed directly from liquid rock), metamorphic rocks (formed by direct alteration of existing rocks), and sedimentary rocks (formed by eroded materials from other rocks). (http://earth.rice.edu/mtpe/geo/geosphere/topics/rocks_a.html)
If you think of a cookie as a rock, the flour, sugar, and chocolate chips are like the minerals that make up the rock. Depending on the recipe, you get different kinds of cookies. It is the same with rocks because each type of rock has a different combination (or recipe) of minerals.
Minerals all have chemical compositions and physical properties unique to that specific mineral. (A chocolate chip in an oatmeal cookie is the same as the chocolate chip in a peanut butter cookie). Even rocks with the same mineral ingredients may be different due to variations in the amounts of minerals (more flour, fewer chocolate chips) and the processes by which they are formed such as being burned, doughy, or just right. Common rock-forming minerals are feldspar, quartz, calcite, mica, and hornblende. (http://scienceviews.com/geology/rockproperties.html)