A Drupal installation comes with a lot of useful modules preinstalled. If you need more modules for more functions in your website you can download free modules from the official Drupal Modules website.
In your website, Drupal modules are saved in the modules subfolder inside the Drupal's installation folder. When you find the desired module you want to install. Download it to your computer's hard drive and then install it by simply extracting the files and uploading them in their own folder inside the modules folder of the Drupal installation.
The other way is to use the automated installation function located at the top of the Modules page > Install new module. You can enable or disable a Drupal Module from the Modules page too.
From the Update tab you can see the modules that need an update and from the Uninstall category the disabled modules that can be uninstalled from your system.
Where to pay attention
Adding and removing Drupal modules is a simple process that can add and remove features from a website, but it is necessary that all the checks regarding the impact on the performance and on the security that a module have on your website should be made.
Regarding security, you will need to do a thorough investigation on reviews from other users of the particular module. If the reviews are bad or a few, you should be very careful if you add it to your website or even avoid it if you can find a better alternative module.
Regarding performance, it is advisable to always have in mind that a new module inserts new code in your website which may affect its efficiency. The algorithmic complexity that may be added to your website, the operation on a large set of records in the database and the use of other modules together, produces combination of interacting parts that may not always have the desired results. The same code may work well in a small set of records, but not as good with more records.
The benchmarking statistics availability from the module creator, may be a critirion of its selection or its rejection. It is good to know the behavior of the modules code in a few or many records in the database in order to make the safer decision to add it or not in the website.
Finally, module installation it's good to be done gradually. This way you will be able to check each one individually. If you add a lot of modules together and your websites behavior change, it will be difficult to find exactly which module causes the problem, so prefer to install modules gradually while checking your websites privacy and performance.
