CSS Flex is a layout form used by websites that allow certain items or containers to change or be dynamic based on the size and arangement of other items on the page. The flexbox layout has its advantages and disadvantages compared to other layouts, like the jell-o layout with floats and positioning. Some big advantages are, as listed through the MDN article, "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," and "Making all columns in a multiple-column layout adopt the same height even if they contain a different amount of content." The flex model is laid across two axes, the main or x-axis, and the cross or y-axis. Flex items can also be sized differently, allowing one or more items to take up more space than others while retaining the flex style. Order can also be changed through flex directly isntead of shifting items from the source code. CSS Flex is a great layout to use for the development of any website and has many uses that are advantageous over other layout models!
Flexbox is a form of flex that allows for one-dimensional layout adjustments. Although somewhat restricting, the use of flexbox can allow for containing items in a row or column layout, but not both simultaneously. There's also the flex-wrap property that allows the items in a container to wrap properly instead of extending outside their designated area, thus forcing the horizontal scroll which everyone dislikes. Flexbox is another way to use the idea of flex.
Flexbox, while limited, does have a fair amount of uses. One notable use is for a navigation menu. Nav menus tend to be one dimensional, whether on the y-axis or x-axis. This means they would fit the use of a flexbox layout perfectly, being a one dimensional layout method. Another great use is for pushing a footer down to a certain spot on the page rather than it being where the text cuts off on the last container or item. There are a lot of different applications for the flexbox method, so go ahead and click the link above to find some more uses for flexbox!
To Summarize
As mentioned above, flexbox and flex can be used in a lot of ways. They're especially useful when you want to design a page or site that requires more dynamic layouts and prevents too much empty space. It's also useful when desiring balance, as you can adjust the size and position of flex containers as you or the user(s) see fit.