What kind of drugs can be absorbed through skin?
The skin is able to prevent some things from passing in and out of a person but it is not a perfect barrier.

The lipophilic things like nicotine, hydrogen flouride, VX, hexane and iodine vapour can pass through undamaged skin. These things are able to dissolve in the fatty parts of skin. Then they diffuse in through the skin into the person. Thus they enter the person.

Some water soluble ionic things can enter / leave a person through the skin, this can occur through the sweat pores.

For example cesium in humans does come out in sweat, in a 1980s case in Brazil where a lot of people were internally contaminated with cesium-137. It was found that cesium kept appearing on the surface of the skin of the contaminated people.