Planar backgrounds mean that the background image has been applied to a plane obj and then converted into a prop item so that you can go through your content folders, click on the icon, and it automatically loads. This way you can move the background back or forward depending on your needs. In another words you don’t have to worry abour digging through folders on your hardrive and manually placing a background in. The click of the thumbnail will do it for you.
I don’t have poser so I don’t know how their backgrounds work. For Daz however, if you have a jpg or png image file that you have bought or downloaded to be used as a background then all you have to do is go to edit (at the top of Daz window) and then scroll down to backdrop. When you click on that a popup will appear for you to select the image you want to use. Once you bring the background image in you won’t be able to move it around like you would with a planar background.