Hello,

I have noticed you sent the same request before and my colleague already answered it, so I just paste here her response for other visitors:

Containers are alternative of Content before and Content after fields which are available in each web part. A difference is that you can define code which will be inserted in front of and after web part from one place:
Containers

Web parts have a default layout which you can find in markup of web part code. If you need to change it, you can use layout tab and change the order of contrls in a web part:
Layouts

If I create a grid with two columns web part or two containers, Both are containers of other web parts.

You can create a grid web part with one button for example. In layout tab you can change order of controls, i.e. button can be first, grid second and if you define container you can create a div element which will wrap the code of your web part.

Best regards,
Jan Hermann