First off—what is a 'web-mapping application'? To put it bluntly, it's some type of Internet application that makes use of a map. This could be a site that displays the latest geo-tagged images from Flickr (we'll do this in Chapter 11), a map that shows markers of locations you've traveled to, or an application that tracks invasive plant species and displays them. If it contains a map and it does something, you could argue that it is a web map application. The term can be used in a pretty broad sense.
So where exactly does OpenLayers fit in? We know OpenLayers is a client side mapping library, but what does that mean? Let's take a look at the following screenshot:
This is called the Client / Server Model and it is, essentially, the core of how all web applications operate. In the case of a web map application, some sort of map client (e.g., OpenLayers) communicates with some sort of web map server (e.g., a WMS server or the Google Maps backend).
OpenLayers lives on the client side. One of the primary tasks the client performs is to get map images from a map server. Essentially, the client has to ask a map server for what you want to look at. Every time you navigate or zoom around on the map, the client has to make new requests to the server—because you're asking to look at something different.
OpenLayers handles this all for you, and it is happening via asynchronous JavaScript (AJAX) calls to a map server. To reiterate—the basic concept is that OpenLayers sends requests to a map server for map images every time you interact with the map, then OpenLayers pieces together all the returned map images so it looks like one big, seamless map. In Chapter 2, we'll cover this concept in more depth.
A map server (or map service) provides the map itself. There are a myriad of different map server backends. A small sample includes WMS, Google Maps, Yahoo! Maps, ESRI ArcGIS, WFS, and OpenStreet Maps. If you are unfamiliar with those terms, don't sweat it. The basic principle behind all those services is that they allow you to specify the area of the map you want to look at (by sending a request), and then the map servers send back a response containing the map image. With OpenLayers, you can choose to use as many different backends in any sort of combination as you'd like.
OpenLayers is not a web map server; it only consumes data from them. So, you will need to be able to access some type of web map service. Don't worry though. Fortunately, there are a myriad of free and/or open source web map servers available that are remotely hosted or easy to set up yourself, such as MapServer.
Note
Throughout this book, we'll often use a freely available web mapping service from OSGeo, so don't worry about having to provide your own.
With many web map servers you do not have to do anything to use them—just supplying a URL to them in OpenLayers is enough. OSGeo, OpenStreet Maps, Google, Yahoo!, and Bing Maps, for instance, provide access to their map servers (although, some commercial restrictions may apply with various services in some situations).