When we say client-side, we are referring to the user's computer, specifically their web browser. The only thing you need to make OpenLayers work is the OpenLayers code itself and a web browser. You can either download it and use it on your computer locally, or download nothing and simply link to the JavaScript file served on the site that hosts the OpenLayers project (http://openlayers.org). OpenLayers works on nearly all modern web browsers and can be served by any web server or your own computer. Using a modern, standard based browser such as Firefox, Google Chrome, Safari, or Opera is recommended.

When we say library, we mean that OpenLayers is a map engine that provides an API (Application Program Interface) that can be used to develop your own web maps. Instead of building a mapping application from scratch, you can use OpenLayers for the mapping part, which is maintained and developed by a bunch of brilliant people.

For example, if you'd want to write a blog, you could either write your own blog engine, or use an existing one such as WordPress or Drupal and build on top of it. Similarly, if you'd want to create a web map, you could either write your own from scratch, or use software that has been developed and tested by a group of developers with a strong community behind it.

By choosing to use OpenLayers, you do have to learn how to use the library (or else you wouldn't be reading this book), but the benefits greatly outweigh the costs. You get to use a rich, highly tested, and maintained code base, and all you have to do is learn how to use it. Hopefully, this book will help you with it.

OpenLayers is written in JavaScript, but don't fret if you don't know it very well. All you really need is some knowledge of the basic syntax, and we'll try to keep things as clear as possible in the code examples.