CSS Flex box

 Flex box is a one-dimensional layout method for laying out items in rows or columns. Items flex to fill additional space and shrink to fit into smaller spaces.
Why Flex box?For a long time, the only reliable cross browser-compatible tools available for creating CSS layouts were things like floats and positioning. These are fine and they work, but in some ways they are also rather limiting and frustrating.
The following simple layout requirements are either difficult or impossible to achieve with such tools, in any kind of convenient, flexible way:
• Vertically centering a block of content inside its parent.
• Making all the children of a container take up an equal amount of the available width/height, regardless of how much width/height is available.
• Making all columns in a multiple column layout adopt the same height even if they contain a different amount of content.
An aside on the flex model
When elements are laid out as flexible boxes, they are laid out along two axes:
• The main axis is the axis running in the direction the flex items are being laid out in (e.g. as rows across the page, or columns down the page.) The start and end of this axis are called the main start and main end.
• The cross axis is the axis running perpendicular to the direction the flex items are being laid out in. The start and end of this axis are called the cross start and cross end.

The flex container
An area of a document laid out using flex box is called a flex container. To create a flex container, we set the value of the area's container's display property to flex or inline-flex. As soon as we do this the direct children of that container become flex items. As with all properties in CSS, some initial values are defined, so when creating a flex container all of the contained flex items will behave in the following way.
• Items display in a row (the flex-direction property's default is row).
• The items start from the start edge of the main axis.
• The items do not stretch on the main dimension, but can shrink.
• The items will stretch to fill the size of the cross axis.
• The flex-basis property is set to auto.
• The flex-wrap property is set to no wrap.
The result of this is that your items will all line up in a row, using the size of the content as their size in the main axis. If there are more items than can fit in the container, they will not wrap but will instead overflow. If some items are taller than others, all items will stretch along the cross axis to fill its full size.