Like layers of an onion, each layer is above and will cover up the previous one; the order that you add in the layers is important. With OpenLayers, you can arbitrarily set the overall transparency of any layer, so you are easily able to control how many layers cover each other up, and dynamically change the layer order at any time.

Most of the time, you make a distinction between base layers and non-base layers. Base layers are layers below the others and are used as a background on your maps to give general context. When you choose one base layer, the others will not be shown. On the top of base layers, you have non-base layers used to emphasize particular topics. You can also choose to use only overlay layers if you're considering that they are enough to understand the map. As a classical example, you could have a Bing map as your base layer, a layer with satellite imagery that is semi-transparent, and a vector layer, all active on your map at once. A vector layer is a powerful layer that allows for the addition of markers and various geometric objects to our maps—we'll cover it in Chapter 5, Using Vector Layers. Thus, in this example, your map would have three separate layers. We'll study in detail about layers and how to use and combine them in Chapter 4, Interacting with Raster Data Source.