Lightning for Drupal 7 is essentially a demo at this point (Q2 2016).
Acquia's Demo Framework team created Lightning so that we could have a decoupled, lightweight version of our demo that was more stable.
When Acquia's product team took over, we had hoped to staff a team that could support maintaining both 7.x and 8.x versions, but this was not possible given the resources available. The product team decided to go ahead with Drupal 8 and the Module Acceleration Project.
If you are starting a new site today, we recommend using the Drupal 8 version of Lightning.
The following is legacy information re: Lightning in 7.x -- your mileage may vary. The Demo Framework team still intends to provide basic maintenance of the 7.x branch, but we do not have a release candidate planned.
What is Lightning?
In order to find the Drupal 7 version of this module you must select "View All Releases" and either scroll down the page looking for 7.x-1.0 OR filter the list to the 7.x version.
In Drupal 7, Lightning provided decoupled Features that can be used to accelerate Drupal project building. Most of these packages involve some form of module combination that users commonly need but might have difficulty making work without a lot of trial and error.
The "Lightning" project found here should be thought of as an example of using the Lightning Features with all of the Features turned on for evaluation purposes. This might not be realistic for every project, but its definitely helpful for evaluating what Lightning's packages can offer.
Lightning gives you the recipes to the "best of Drupal" without holding you back
Lightning Features are basically "recipes" of contributed modules working together. These modules are packaged with config to get the functionality you'd expect from an enterprise class CMS. These are standalone, decoupled Features that can work separately and in tandem.
There is a small demo included with the Lightning install profile that you can turn on to help evaluate Lightning's site building offerings. There are no other modules versioned within the Lightning profile. Instead, our Features are their own project that you can add to your existing builds.
More on Lightning Features
The decoupled Lightning Features are provided as their own project. Each one can be turned on or off without incident because they are not wound together in a rat's nest of dependencies. You can find them here: http://drupal.org/project/lightning_features
Using Lightning is a paradigm shift compared to other distros
In most cases, you will not use the full Lightning distribution when you are building a project "on top of Lightning." If you want to see an example of a site built on top of Lightning, consider the Demo Framework. The Demo Framework has its own profile and its own drush make files. It declares lightning_features as a dependency and moves on from there. It uses a large portion of the Lightning Features, but it doesn't use everything provided.
I want to build my site using Lightning, what should I do?
Consider forgoing the profile that ships with Lighting and only using the Lighting Features you actually need for individual projects. Your life will be much easier if you continue developing sites the way you are comfortable, but lean on Lightning Features to provide the correct recipes for complex combinations of modules.
For example: You don't want to deal with the complexities of what versions of Panels, Panelizer, Fieldable Panels Panes and Ctools are needed to allow for fully revisionable layout management to "just work" on the first try. That's a recipe Lightning Features provides. We call that one "lightning_panels" - check it out.
It's a distro, without the usual distro baggage
Lightning Features provides module combinations as decoupled packages. If you choose to use these you can reap all the benefit you'd expect from a Drupal distribution, without the headaches that normally come with trying to customize on top of them. You won't need to fork Lightning or worry about complex distribution updates or "apps" services. You can include Lightning Features as part of your project and choose what to use and what not to.