The Flex Box Module usually referred to as flexbox, was designed as an one-dimensional layout model.
It make easier to design flexible responsive layout structure without using float or positioning.

Here is a demo to play around about CSS Flex Box Module: https://shafikshaon.github.io/css-flexbox/

The flex container properties are:

• flex-direction
• flex-wrap
• flex-flow
• justify-content
• align-items
• align-content

flex-direction

The main axis defined by the flex-direction property. It defines in which direction the container want to stack the flex items.

The flex-direction has four possible values:

• row
• row-reverse
• column
• column-reverse

row: The row value stacks the flex item horizontally but from left to right.

row-reverse: The row value stacks the flex item horizontally but from right to left.

column: The column value stacks the flex item vertically but from top to bottom.

column-reverse: The column value stacks the flex item vertically but from bottom to top.

flex-wrap

Flex box is one dimensional model. So, it is possible to cause our flex items wrap into multiple lines. We should consider each line as new flex container.

flex-wrap property specifies whether items should wrap or not.
Wrap means cover or enclose (someone or something) in paper or soft material.

wrap: The wrap value specifies that the flex items will wrap if necessary.

nowrap: The wrap value specifies that the flex items will not wrap.

wrap-reverse: The wrap value specifies that the flex items will wrap if necessary but in reverse order.

flex-flow

flex-flow is shortland for both the flex-direction and flex-wrap properties

for example

.class {
    flex-flow: row wrap;
}

justify-content

The justify-content property is used to align the items on the main axis where direction determine by flex-direction.

flex-start: flex-start value align flex items at the start of the container.

flex-end: flex-end value align flex items at the end of the container.

center: center value align flex items at the center of the container.

space-around: space-around value align flex items with space before, between and after the lines. It causes equal amount of space on the right and left of each item.

space-between: space-between value align flex items with space between the lines.

space-evenly: space-evenly value align flex items with full-size space on either end.

align-items

The align-items property used for align flex items vertically.

stretch: stretch value stretch the flex items to fill the container. This is why flex items stretch to the height of the tallest one by default.

baseline: baseline value align the flex items by their flex container baselines.

center: center value align the flex items to center of the container.

flex-start: flex-start value align the flex items to top of the container.

flex-end: flex-end value align the flex items to bottom of the container.

align-content

The align-content property used for align flex lines. To demonstrate the align-content property set flex-wrap property set to wrap.

stretch: stretch value stretches the flex lines to take up the remaining space.

center: center value show the flex lines in the center of the container.

flex-start: flex-start value show the flex lines in the start of the container.

flex-end: flex-end value show the flex lines in the end of the container.

space-around: space-around value show flex lines with space before, between and after the lines. It causes equal amount of space on the right and left of each lines.

space-between: space-between value align flex lines with equal space between the lines.

Now we know about flex item properties:

order: order property specify the order of the flex items. The order value must be a number, default value is 0.

<div class="flex-container">
  <div style="order: 3">1</div>
  <div style="order: 2">2</div>
  <div style="order: 4">3</div>
  <div style="order: 1">4</div>
</div

flex-grow: flex-grow property specifies how much flex item glow (increasing in size) relative to other flex items. The value must be a number, default value is 0.
The flex-grow property can be used to distribute space in proportion.

<div class="flex-container">
  <div style="flex-grow: 1">1</div>
  <div style="flex-grow: 1">2</div>
  <div style="flex-grow: 8">3</div>
</div>

flex-shrink: flex-shrink property specifies how much a flex item will shrink relative to other flex items. The value must be a number, default value is 1.
Do not let the second flex item shrink as much as the other flex items:

<div class="flex-container">
  <div>1</div>
  <div style="flex-shrink: 0">2</div>
  <div>3</div>
</div>

flex-basis: flex-basis values specifies the size of item in terms of space it leaves as available space.
Set the initial length of the second flex item to 20 pixels:

<div class="flex-container">
  <div>1</div>
  <div style="flex-basis: 20px">2</div>
  <div>3</div>
</div>

flex: flex property is a shorthand property for the flex-grow, flex-shrink, and flex-basis properties.
Make the first flex item growable (1), shrinkable (1), and with an initial length of 20 pixels:

<div class="flex-container">
  <div style="flex: 1 1 20px">1</div>
  <div>2</div>
</div>

align-self: align-self property specifies the alignment for selected item inside flex container.
Align the third flex item in the middle of the container:

<div class="flex-container">
  <div>1</div>
  <div>2</div>
  <div style="align-self: center">3</div>
  <div>4</div>
</div>

Here is a demo to play around about CSS Flex Box Module:

https://shafikshaon.github.io/css-flexbox/