Here is a simple paragraph placed inside the BODY element. And outside the DIV that follows. Note the width of the P corresponds to the BODY.
Here is a paragraph placed inside the DIV element. The paragraph width adjusts to fill the width of the DIV. The resulting paragraph width depends on the padding and border widths, and on the margins.
Here is a simple paragraph placed inside the BODY element. The paragraph has a left margin of -5%, which pushes the left side of the paragrah half way back to the left edge of the BODY (since the DIV margin was +10%).