Modules in Drupal are the easiest way to increase the functionality of your site. They are lines of code written in a special way so that they easily slot into your core Drupal system and can be administered through the administration menu. The number of modules you can add is fairly mind boggling and is a sign of the strength of the Drupal community. The modules reflect that when people write an addition to Drupal system they often want to give that work back to the community. The best way to do this is to package up that additional functionality into a module and put it on the Drupal website for others to download, evaluate, bug test, revise etc.
Evaluating and Choosing Modules
The process of researching which module to use is a skill in itself. Mature and stable modules quickly and easily add great functionality to your site, however not all modules have been tested extensively and some are what we would call 'bleeding edge' technology. The last thing you want to do is to waste time and potentially break your site temporarily by installing a module that in all honesty isn't really ready to be used.
There are a few ways to check how effective your module is and how ready it is to use none of these are exhaustive but . You can have a look at the comments at the foot of the module's home page. There are also links for Resources, Support and Development. The Read Documentation page is a good start for your research.
Looking at the contents of these links is a good way to see if the project is active or not. Although activity is not necessarily a sign that that module is right for you, as a simple, stable module may not be very active. You can click on View all issues to see how many bugs are being reported, what feature requests there are and how responsive the community of developers are. You'll see a screenshot as below.
There are different ways of installing modules. The end result of all the ways of installing modules is that the module which is distributed as a .tar.gz - 'tarball' file, is extracted and the resulting folder or files placed in the /sites/all/modules directory of your Drupal installation.