Along the same lines as this question. What's so special about enabling Unknown Sources, and what are some things that I should do once it's enabled?
This allows you to obtain and run an application (i.e. the .apk file) from sources other than the Android Market. (i.e. you get it from a download from a developer's site, from an email, etc.). That's it.
Edit: Once you enable Unknown Sources this post covers the rest: How can I install an app given only its APK file?.
What's so special about enabling Unknown Sources?
It opens the secondary gateway to allow third-party apps to live and prosper in your Android OS. The first, centralized and trustworthy gateway is the Google Play Store.
That setting allows developers to use (not limited to) the following channels for the distribution of their apps:
- through an App Marketplace, such as Amazon AppStore, F-Droid and many more
- by Email - when the user open your email on their Android-powered device, the Android system recognizes the APK and displays an Install Now button in the email message
- through a Website - this involves forums as well.
Source: Alternative Distribution Options
The most common ways to install an app are:
- from a system app, such as Google Play Store
- from adb, the command-line tool
- from native Package Installer app - works only when you enables that Unknown sources setting
- from exploits - the bread-and-butter of one-click root apps
As you can see, the first choice ties you down to whatever was shipped with the device; the second choice is not user-friendly and requires PC; fourth is an unsafe choice from any angle; third is the only one left to a user. Special or not, that is what gives you the freedom to install apps, not found or not available in your region per Play Store, whenever and wherever you want.
Note that some network providers don’t allow users to install applications from unknown sources. -- Source
It is to be noted that it is special not only to you but to developers with malicious intent as well.
Elena, in the article Protect against harmful apps, has noted:
IMPORTANT: Your phone and personal data are vulnerable to attack by apps from unknown sources. If you allow downloading apps from unknown sources, be aware that your phone may be damaged or lose data.
I suppose that warning stems from their strong believe in the following statement found in the same article:
The Google Play Store helps protect your device by blocking potentially harmful apps. Before an app is published to Google Play, it's reviewed to make sure it's not harmful.