A media asset is anything you can embed in your site's content. This can include things like images, videos, audio clips, slideshows, tweets, and PDF documents. It's possible to define your own, exotic media types as well. Lightning includes tools that just freakin' work and make it drop-dead stupid simple for you, as a content author, to embed any kind of media into your content.

By default, all media assets are stored in your site's media library. The media library is searchable and keeps track of all kinds of media.

When authoring a piece of content, you can always insert a media asset by clicking the Media Library button in the editor:

The Media Library button in the CKEditor toolbar. It's the flashy star-shaped one :)

This will call up the ten most recently-added assets in media library, newest first. More are revealed as you scroll down. You can also search by keywords, although at the time of this writing it's only able to search for partial matches in the filenames of uploaded images.

Searching for partial matches in a hidden filename is pretty lame, but this will be fixed up in the future.
The glorious media library. And there was much rejoicing.

So go ahead — click on an asset in the media library to select it, click the "Place" button, and boom — it gets embedded right into your content. Slick, no?

The asset has to be fetched from the server, so there may be a delay of a few seconds before it appears. Don't panic.
Content always looks better with media assets thrown in.

You can also use the Media Library button to upload a new image into your library. Open the Media Library and select the Upload tab. The nondescript grey box is your upload widget. You can drag and drop an image onto it, or click it to browse for an image on your hard drive. When an image has been dropped on it (or chosen), it will be uploaded to the server. Once uploaded, click "Place" to embed it into your content.

Optionally, you can check the "Save to media library" checkbox beforehand if you'd like to put the image into your media library for future use. If you save the uploaded image to your library, it will be the first thing in the library next time you open it.

Currently, you can only upload images this way. Support for additional types of uploads (PDF documents, for example) will be added in the future.