What is Salesforce Mobile SDK 3.1 & Why to use it?

Quick characteristics of Salesforce Mobile SDK 3.1: 

1. It is unified app architecture (SmartSync API for fast and responsive apps) brings unparalleled flexibility meaning that you can built hybrid or native apps using same core set of functionality regardless of their target platform. 
2. Salesforce Mobile SDK 3.1 does have everything consistent like all the libraries, all the APIs & all the major mobile building blocks.
3. Extended choice of development tools or UI frameworks like for iOS you can now use CocoaPods. So, with the Salesforce Mobile SDK 3.1 release, mobile developers no longer have to make these trade offs for selecting a preferred technology.
4. Salesforce Mobile SDK 3.1 have flows to initiate authentication with an enterprise certificate.

Something Developers need to keep in mind!

1. iOS 7 is now the minimum OS version for developing your iOS apps on the top of Salesforce Mobile SDK 3.1.
   
2. For Android, min version required is 4.2.2 - API_17

Unified SmartSync APIs : This gives "Bulk Sync Routines Executive" from the native layer, even in hybrid apps with a new SmartSyncPlugin for Cordova apps. It also helps for full CRUD operations for online and offline usage.

CocoaPods for iOS : Developers now have the ability to consume the Salesforce Mobile SDK through CocoaPods, which makes dependency management a chore of the past. It’s now easier than ever before to insert Salesforce Mobile SDK modules into an existing app or evolve a new app with other third party libraries.

Gradle for Native Android Apps : Android Studio and the Gradle build system are now fully supported for native apps.  Gradle support for hybrid apps will arrive after the next Cordova update.

Certificate Based Authentication Pilot : Leverage certificates as part of an enhanced authentication flow without asking your users to enter a username and password.

Some Important Links

1. iOS Native SDK: https://github.com/forcedotcom/SalesforceMobileSDK-iOS/tree/master/native/SampleApps/SmartSyncExplorer
2. Android  Native SDK: https://github.com/forcedotcom/SalesforceMobileSDK-Android/tree/master/native/SampleApps/SmartSyncExplorer
3. Hybrid App Native SDK: https://developer.salesforce.com/blogs/engineering/2014/11/cross-platform-apps-salesforce-mobile-sdk-web-components.html

References:

1. https://developer.salesforce.com/blogs/engineering/2015/02/salesforce-mobile-sdk-3-1-unified-app-architecture-brings-unparalleled-flexibility.html
2. https://developer.salesforce.com/page/Building_Native_iOS_Apps_With_the_Salesforce_Mobile_SDK_3.1
3. https://developer.salesforce.com/page/Mobile_SDK