The foxycart_includes.js file is specific to your store version (as specified in your store settings). It uses Dean Edward's Packer to compress a few other files that help make it easier to implement FoxyCart.
jquery.js: We're currently using jQuery. It's in “no conflict mode” so your other libraries will work fine.
form.js: We're using malsup.com's form.js to assist with sending form data to your cart.
foxycart.js: This file contains the code necessary to use the JSON object, as well as some other code specific to FoxyCart.
foxybox.js: “FoxyBox” is simply a modified version of Cody Lindley's very cool Thickbox. FoxyCart is currently using version 2 of Thickbox.