purpose of new d-flex class in bootstrap

what is major difference between

<div class="d-flex flex-row
<div d-flex flex-column

vs regular

<div class="col" and <div class="row"

construct. do we use the flex-row and flex-column construct, similar to bootstrap 3 , or these are for totally different purpose.
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Julian HansenCommented:
Take a read of these to links
A comprehensive guide to FlexBox
The Bootstrap Flex Classes Guide

With flex box you can arrange your content in rows or columns and then have the flex system expand / fill your content to the available space.

So very simply flex-row is bootstraps style for flex-direction: row
and flex-column is Bootstrap's style for flex-direction:column

It just means that the content must (in the case of a row) be placed next to each other and (based on other flex options) expanded / shrunk to fit the available space or just sit side by side.

For flex-column it is the same but items are stacked vertically.

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