Traditional network applications have both server and client side, each of which is a program in its own right.
As a very simple example, consider a networked game, where you are playing against a number of other people. The server holds a database of the present positions of everything, and decides if something is allowed or not. On each player's computer, there is a client, which gets data from the server and displays it in a user-friendly way, such as rendering a 3D world that you can move around in. The client is smart enough that if you try to do something that is obviously not allowed, such as run through a wall, then it won't let you.
It would be silly if a 3D screen was generated on the server and was sent to the client 24 times per second. And it would be awkward if you tried to do something like walk through a wall, and had to wait for the server to decide if it was allowed or not.
However, in a pure PHP environment, that's exactly what has been happening—the server generates absolutely everything that the client displays. If an element needs to be removed from a table, for example, then the server needs to be told, so that it can give the client the new view. If you try to do something illegal, such as submit a blank form where an email address is required, then pure PHP applications will let you do that, wasting time and server resources.
However, using JavaScript, and especially using Ajax, the client side is no longer a "dumb terminal" for the server. We can write applications fully on the client side, if we wish, using the server purely as a database. This book will demonstrate ways to work towards that.
Although Ajax stands for Asynchronous JavaScript And XML, it's a misnomer. This is because in most cases, no XML is involved (either plain HTML or data in JSON format is usually used). Ajax allows a client to open a data channel between itself and the server, allowing JavaScript to load new information from the server without needing to refresh the entire screen.
JSON stands for JavaScript Object Notation. It is a simple data storage format, which can be used to transfer information about strings, numbers, arrays, and associative arrays between various languages. It is much smaller than XML, and is much easier to transform because there is a direct relationship between the text format and the internal data format. It can be used in both PHP and JavaScript. It's based on how JavaScript is actually written, so it can be natively compiled into an object in JavaScript.
In JavaScript, you can create an actual data object from JSON by simply "eval"ing it (or using the new json.decode function in newer browsers). In PHP, you can use the json_decode and json_encode functions to convert from and to JSON.
With Ajax, we can provide a client-side experience, which had only been possible beforehand using external plugins, such as Java or Flash. It could not safely be assumed that these plugins were available on the client. Every major browser now supports Ajax, so there are much fewer reasons to rely on proprietary technology these days.
jQuery simplifies Ajax. With pure Ajax, you need to do low-level stuff like create an XMLHTTPRequest object (taking care of cross-browser incompatibilities), set callbacks, check response values, and so on. With jQuery, all you need to do is use $.get or one of the other jQuery JSON functions. These functions will take care of the low-level stuff for you.