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.

Breaking down the definition of "Signal"

I'm at a new job unexpectedly working with AV and was thinking this wasn't my scene. However, it turns out all the devices are now networked and there's a large digital component to the systems that I was unaware of. In result, I'm learning the lingo and inner workings of this world.

I don't come from an electrical engineering background, so when someone says "signal" I'm wary of what they could possibly be referring to. Wikipedia refers to it as a function which conveys behavioral or characteristic information of a phenomena (not word for word).  This is less vague than I expected but still kind of vague. When it says function, does it mean a mathematical function or like a function found in OOP, or generally the function of some sort of a tool. The final one doesn't seem right simply because it's not being used in the sentence correctly for that version. I'm just going to ignore this word for now. It is something that conveys information about something that happens in nature. How about that?

Furthermore, Wikipedia claims that any quantity that can be measured concerning a variation in time or space may be a signal since it will provide information about a physical system. This, I think is more telling of what a signal actually, effectively is.

Yay for clarifying ambiguous engineering terms!