Nice write up and very cool shots!
Here's the basics:
Aperture is the adjustable opening that lets light in the lens. They are measured in f-stops. Higher f-stop means a smaller aperture, lower f-stop is a larger aperture. So, f/2.8 is very wide while f/16 is narrow. Aperture controls depth of field (how much of the photo is in focus). The wider the aperture (smaller f-stop), the less there is in focus. This is good for separating subjects from the background. It will make your subject sharp while your background is blurry. The smaller the aperture, the more there is in focus. This is good for landscapes. Keep in mind that the image will actually start to get fuzzy past f/16, so that's usually as far as I push it. The aperture also affects how much light goes through the lens and onto the sensor, so as you change the aperture, you'll need to adjust shutter speed. Wider apertures let in more light, which means a faster shutter speed. Narrower apertures let n less light, so it will slow down the shutter speed.
Shutter speed controls how long the sensor is exposed to light. It also manages how motion appears in the photos. Faster speeds will freeze motion, slower speeds will blur it. Set the shutter speed too low, and you'll see camera shake in the photo. A good rule of thumb to avoid this is to always shoot at or above the lens focal length (what mm the lens is). So, at 18mm, only shoot at or above 1/20 sec. At 55mm, shoot at or above 1/60 sec. This rule does depend on how steady your hands are. Shutter speed is usually what gets adjusted to adapt to lighting changes (it gets cludy, the sun sets, you go inside, or you change the ISO or aperture).
ISO controls how sensitive the sensor is to light. Higher ISO is more sensitive, lower ISO is less sensitive. 100 or 200 ISO is good for being outside during sunlight, 400 is good for clouds, early morning, or late afternoon. 800 is for dawn, dusk, or indoors. The darker it gets, the higher the ISO should go. Keep in mind that as you push the ISO higher, you introduce more digital noise. Noise will make the image less sharp, add splotches of color, and make the image less crisp. Take an image at 100 ISO, then take it at 3200 or 6400 to see what I mean. If you have an aperture you like, but you can't get a fast enough shutter speed, bump the ISO up. This will let you get the faster shutter speed.
In manual mode, the light meter will help you balance these settings. If you want to experiment with depth of fueld effects and figure out ISO, set the camera to aperture priority mode (A mode).This mode allows you to set ISO and aperture while the camera works out shutter speed.
Hope that helps!