The Flexbox Layout (Flexible Box) module (currently a W3C Last Call Working Draft) aims at providing a more efficient way to lay out, align and distribute space among items in a container, even when their size is unknown and/or dynamic (thus the word "flex").
The main idea behind the flex layout is to give the container the ability to alter its items' width/height (and order) to best fill the available space (mostly to accommodate to all kind of display devices and screen sizes). A flex container expands items to fill available free space, or shrinks them to prevent overflow.
Most importantly, the flexbox layout is direction-agnostic as opposed to the regular layouts (block which is vertically-based and inline which is horizontally-based). While those work well for pages, they lack flexibility (no pun intended) to support large or complex applications (especially when it comes to orientation changing, resizing, stretching, shrinking, etc.).
Flexbox layout is most appropriate to the components of an application, and small-scale layouts, while the Grid layout is intended for larger scale layouts.
Flexbox requires some vendor prefixing to support the most browsers possible. It doesn't just include prepending properties with the vendor prefix, but there are actually entirely different property and value names. This is because the Flexbox spec has changed over time, creating an "old", "tweener", and "new" versions.
Perhaps the best way to handle this is to write in the new (and final) syntax and run your CSS through Autoprefixer, which handles the fallbacks very well.
Alternatively, here's a Sass @mixin to help with some of the prefixing, which also gives you an idea of what kind of things need to be done:
(new) means the recent syntax from the specification (e.g. display: flex;)
(tweener) means an odd unofficial syntax from 2011 (e.g. display: flexbox;)
(old) means the old syntax from 2009 (e.g. display: box;)
20- (old) 21+ (new)
3.1+ (old) 6.1+ (new)
2-21 (old) 22+ (new)
10 (tweener) 11+ (new)
2.1+ (old) 4.4+ (new)
3.2+ (old) 7.1+ (new)
While Blackberry browser 10+ supports the new syntax.
For more informations about how to mix syntaxes in order to get the best browser support, please refer to this article (DevOpera).
Basics & Terminology
Since flexbox is a whole module and not a single property, it involves a lot of things including its whole set of properties. Some of them are meant to be set on the container (parent element, known as "flex container") whereas the others are meant to be set on the children (said "flex items").
If regular layout is based on both block and inline flow directions, the flex layout is based on "flex-flow directions". Please have a look at this figure from the specification, explaining the main idea behind the flex layout.
Basically, items will be laid out following either the main axis (from main-start to main-end) or the cross axis (from cross-start to cross-end).
main axis - The main axis of a flex container is the primary axis along which flex items are laid out. Beware, it is not necessarily horizontal; it depends on the flex-direction property (see below).
main-start | main-end - The flex items are placed within the container starting from main-start and going to main-end.
main size - A flex item's width or height, whichever is in the main dimension, is the item's main size. The flex item's main size property is either the ‘width’ or ‘height’ property, whichever is in the main dimension.
cross axis - The axis perpendicular to the main axis is called the cross axis. Its direction depends on the main axis direction.
cross-start | cross-end - Flex lines are filled with items and placed into the container starting on the cross-start side of the flex container and going toward the cross-end side.
cross size - The width or height of a flex item, whichever is in the cross dimension, is the item's cross size. The cross size property is whichever of ‘width’ or ‘height’ that is in the cross dimension.
List of properties for the Flex Container (Parent)
This defines a flex container; inline or block depending on the given value. It enables a flex context for all its direct children.
Values: flex | inline-flex
display: flex | inline-flex;
Note that CSS columns have no effect on a flex container.
This establishes the main-axis, thus defining the direction flex items are placed in the flex container. Flexbox is (aside from optional wrapping) a single-direction layout concept. Think of flex items as primarily laying out either in horizontal rows or vertical columns.
row - left to right in ltr; right to left in rtl
row-reverse - right to left in ltr; left to right in rtl
column - same as row but top to bottom
column-reverse - same as row-reverse but bottom to top
This defines the alignment along the main axis. It helps distribute extra free space left over when either all the flex items on a line are inflexible, or are flexible but have reached their maximum size. It also exerts some control over the alignment of items when they overflow the line.
flex-start - items are packed toward the start line.
flex-end - items are packed toward to end line
center - items are centered along the line
space-between - items are evenly distributed in the line; first item is on the start line, last item on the end line
space-around - items are evenly distributed in the line with equal space around them. Note that visually the spaces aren't equal, since all the items have equal space on both sides. The first item will have one unit of space against the container edge, but two units of space between the next item because that next item has its own spacing that applies.
space-evenly - items are distributed so that the spacing between any two items (and the space to the edges) is equal.
This defines the default behaviour for how flex items are laid out along the cross axis on the current line. Think of it as the justify-content version for the cross-axis (perpendicular to the main-axis).
By default, flex items are laid out in the source order. However, the order property controls the order in which they appear in the flex container.
order: <integer>; /* default is 0 */
This defines the ability for a flex item to grow if necessary. It accepts a unitless value that serves as a proportion. It dictates what amount of the available space inside the flex container the item should take up.
If all items have flex-grow set to 1, the remaining space in the container will be distributed equally to all children. If one of the children has a value of 2, the remaining space would take up twice as much space as the others (or it will try to, at least).
flex-grow: <number>; /* default 0 */
Negative numbers are invalid.
This defines the ability for a flex item to shrink if necessary.
flex-shrink: <number>; /* default 1 */
Negative numbers are invalid.
This defines the default size of an element before the remaining space is distributed. It can be a length (e.g. 20%, 5rem, etc.) or a keyword. The auto keyword means "look at my width or height property" (which was temporarily done by the main-size keyword until deprecated). The content keyword means "size it based on the item's content" - this keyword isn't well supported yet, so it's hard to test and harder to know what its brethren max-content, min-content, and fit-content do.