Q: What is the difference between an "extension" and a "modification"?
The term "extension" is what OpenCart officially calls addons that are listed in the admin area under the "Extensions" menu (Modules, Payment, Shipping, Totals, Feeds, etc). Extensions are dynamic in design and are usually drop-in without having to overwrite or edit any files. They also are usually not likely to be overwritten during upgrades since they are all self contained. A "modification" is different than an extension as it typically means you need to manually edit parts of the core files and these can be overwritten during an upgrade. The terms "addon" and "mods" simply refers to any of my customizations, including both extensions and modifications. Nowadays, most developers use "vQmod" which allows all changes be made in an external script and only temporarily applied during page load. This means files no longer need to be altered manually and upgrades no longer break customizations. More info about vQmod here.
Q: I downloaded the mod but I'm not sure what to do from here.
A: All of my addons come in "zip" format. Most operating systems include a zip program by default. You can right click on the zip file to unzip/extract. Follow the steps by your extraction program to find the extracted files. Once the files are extracted, the directory structure of the files always matches the directory structure of your online store. Using an FTP client like FileZilla, you can see your store files on your webserver. The root path of your store should look like this:
In most extracted addons you will see a similar but smaller folder structure. Usually just two:
Those directories hold all the modifications in the correct directory structure already. So you simply need to "merge" the folders into your FTP client and drop them into some whitespace area (not on top of existing files, but off to the side).
Here are some videos to visualize the process: http://qphoria.blip.tv/posts?view=archive
Q: I think I installed an extension correctly but I don't see it listed in the admin extensions area
A: OpenCart uses a dynamic directory scan to show extensions in the admin area. This means if the controller file exists in the admin/controller/<extensiontype>/ directory, it WILL show. If you are not seeing it listed, then that means the file was not uploaded to the correct directory. If you follow the filecopying videos above you should not run into this.
Q: I see other mods that are similar to yours in the core or from other people. Are they selling the same code?
A: It is entirely possible and legal for 2 people to have the same idea. All of the products on my site are authored by me. I do not resell anyone elses code and I do not allow anyone else to sell mine. Any similar products are other people's attempts at the same type of addon and may be better, worse, or as good as mine. Keep in mind that as an OpenCart core developer, I am quite staked into this business and always keep my modules up to date. More info on differences here