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?.
Visit m.getjar.com and go nuts! download cranky birds. It seems to be the game so many people like.
I wonder if there is an android version of Cut the Rope out yet.
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:
Source: Alternative Distribution Options
The most common ways to install an app are:
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:
I suppose that warning stems from their strong believe in the following statement found in the same article: