"what is the difference between a skin and a container? "

Skins are simply the look and feel for the page, while containers are the look and feel for the modules.

"what is the difference between admin and portal? "

In Site Settings, you can set the Portal Skin and Container, for all normal pages. Setting a different Admin Skin and Container will give you a visual reference when you are on a Admin page or module settings page. You can also add helpful administrative links/info into your Admin skin that normal users would never see.

"and what is the difference between host and site?"

If you are referring to Admin > Site Settings versus Host > Host Settings, then Admin Settings are simply the settings for that portal, while Host Settings is for the "host" and all portals under the host. If you are referring to Skins under Admin and Host, then adding skins or containers under Host > Skins will allow all portals in the installation to have the ability to use all skins/containers installed under Host for that DNN installation. Installing skins and containers under Admin  > Skins will allow only that specific portal to use the skins and containers installed. Other portals in the same DNN installation will not have access to skins and containers installed through Admin > Skins for that site.

Hope that helps!