· Installation profiles are what a developer creates as the basis of distributions. They define installation steps (such as enabling modules, defining content types, etc.) that run after Drupal's base installation when you first install Drupal. One or more standard installation profiles are included in the Drupal Core download; developers can create custom profiles that set up Drupal for specific purposes, and optionally release them for community use on Drupal.org. It is not always easy to attempt to use an installation profile directly, if it requires non-core modules, themes, or libraries -- you would have to locate and download all the required components yourself before you could install Drupal. Instead, it's a lot easier to download a full distribution (if available).
· Distributions are full copies of Drupal that include Drupal Core, along with additional software such as themes, modules, libraries, and installation profiles. The automatic packaging scripts on Drupal.org turn installation profiles into distributions, by gathering all the modules, themes, and libraries they require into a single zip archive, so that all you need to do is download the full archive and run the install script.
When to use distributions
There are no hard and fast rules about when to use distributions, but here are a few guidelines:
· Evaluating Drupal: If you're just getting started with Drupal it makes sense to try a distribution, since they are easier to setup and you can see real life examples of what Drupal can do. Not all distributions are equal though, so start with a popular well maintained distribution.
· Demoing Drupal: Before building a site for someone it can be useful to show them examples of how Drupal can be configured.
· Learning Drupal: In the administration section of a distribution, you can study the inner workings of a real example of Drupal site, learn all the details about how it is built and configured to obtain specific functionality, experiment with any changes and additional modules and themes, etc.
· Quickly building a site: If you're building a site similar to one provided by a distribution it makes sense to start with a distribution. After installing you can continue to configure the site, add modules, create themes, etc. You can even undo things that the installation profile script may have done. If you're changing too much though it may make more sense to just start with stock Drupal and build from there, rather than try to undo and change what was setup for you.