Directory vs. Database

I finished that last AV job. It was a temporary position. I liked it well enough in the way that I enjoyed learning what I do not want to do with my life. The people were nice, the work was all right, but I want something far more social engaging, physically active, and mentally challenging. With that, I am job searching. I keep finding a lot of interesting jobs that I feel I would do well in -and all of them require knowledge of directory services and/or Active Directory specifically. This seems to be a giant, gaping whole in my knowledge of computers, so here I am to figure it out.

What is a directory service?

Initially it always seemed like a directory was a form of a database, but the article I'm trying to learn from makes it seems like directories and databases are distinct things. They mark out a few differences in the construction and use in order to demonstrate to the student how it differs. I will try from memory first to outline these differences then fix it afterwards.

Hierarchal construction in directories vs. geometrical construction of databases:

Perhaps with this non-sense it shows I'm still trying to figure out databases, too. I also couldn't figure out a word to describe what I know of databases' non-hierarchal, but also non-chaotic nature. I see them as blocks of information and then when you get into relational dbs, you start to draw lines in the diagrams when drawing your dbs designs, thus...geometrical *shrugs* It's all I got haha. Directories are hierarchal though from what I understand in that you have an information type and then you have subtypes of that information -kind of like an outline consutrcted during the plot/story design of a book or essay (and unlike this blog which is non-hierarchal, and most definitely chaotic).

Interaction via protocol (directories) vs. API (database)

Directories specifically are good for sharing information between various applications for use in a variety of contexts. Databases are designed and controlled with an API of some language and thus are application specific.

Those are the only differences I can remember. Also this does not answer the initial question -what is a directory *service*? This begs the question...are there database services? Services are another huge part of computing that I know almost nothing about -perhaps I should be exploring what this is first.