It’s old way of building layouts of all sizes, using the float CSS property. Still used, but getting outdated fast.
Modern way of laying out elements in a 1-dimensional row without using floats. Perfect for component layouts.
For laying out element in a fully-fleged 2-dimensional grid. Perfect for page layouts and complex components.
Final element width = right border + right padding + width + left padding + left border
Final element height = topborder + top padding + height+ bottom padding + bottom border
Final element width = width
Final element height = height