Read here, for example:
Try other searches for "voltage divider vs voltage regulator".
eg.
http://www.wd5gnr.com/basiccir.htm
Practical Voltage Division
Don't fall prey to the temptation to use a voltage divider instead of a voltage regulator. Voltage dividers are totally dependent on their supply voltage, so don't try to "regulate" 12V to 5 with a divider. However, there are cases where you might want to use a divider. For example, perhaps you have a 5V A/D converter, but you need to measure 0-10V with it. Of course, you will lose precision, but maybe that's OK.