Accordions are very similar to tabs. They show a single pane of information at a time, and you can navigate through the panes by clicking on a header link for the pane you want to see.
There are some differences between how a jQuery accordion is defined and how a jQuery tab is defined. These implementation differences can be understood by considering how the information should be shown when JavaScript is disabled in the browser—tabs are discrete sections of information (like chapters, maybe), but accordions are usually used only to make the continuous text easier to read.
Again, the jQuery UI project already has a good implementation of accordions built into its core files. So, you don't need to download an extra plugin. But if you wish to use non-standard tricks, such as horizontal accordions (versus vertical, which is the norm), then you should look through http://plugins.jquery.com/ to decide on what exactly you need.
In tabs, the HTML was arranged such that...