When we say client side we are referring to the user's computer, specifically their web browser. The only thing you need to have 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 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 an API (Application Programmer Interface) that provides you with tools 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 wanted to write a blog you could either write your own blog engine, or use an existing one such as WordPress or Blogger and build on top of it. Similarly, if you wanted to create a web map, you could 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 that.

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.