Firefox Aurora for Android is a pre-beta browser release of Firefox for Android.
Firefox Aurora provides a preview into the changes for Firefox for Android. The biggest addition to Firefox Aurora is the ability to install web applications. They work just like regular apps but rely on Firefox Aurora's rendering engine. This creates an ecosystem between Mozilla's products.
Given that this is a pre-release, Firefox Aurora is very laggy and buggy. The browser loads web pages quickly enough but using apps from the Firefox Marketplace really slows the program down. While there Firefox Aurora didn't crash much, there is considerable lag navigating its interface.
Applications from the Firefox Marketplace work well and act as stand-alone apps, placing an icon on your Android's homescreen. There aren't many apps available in the marketplace but that will change as Mozilla grows its app store.
Overall, Firefox Aurora provides a usable preview into the future of Firefox on Android but it's lag will prevent it from becoming your default browser.