I'm also NOT a chemmical guy, but in simple terms...
Both are methods of separating one media from another.
An absorber will gather/coalesce a media by soaking it up, similar to the way a paper towel will pick up water.
An adsorber has a media in the vessel that will cause another media to cling to the surface of it.
The difference being one will gather the media by soaking it up, the other will gather media by having it cling to it's surface.
When do you use one vs the other?? Not sure, that question is more suited to a chemical person. Hope that helps.