Foundation

Foundation is basically the Apple development standard library, implemented in Objective-C. It includes data types like NSArray, NSString, and NSDictionary. However, Swift provides replacements for these data types: Array, String, and Dictionary. You should only need to import Foundation in files that try to interact with Objective-C APIs. However, since all existing APIs are written in Objective-C (since Swift is so new), you will have to import Foundation in almost all files except for basic domain objects.

UIKit

UIKit actually includes Foundation. If you import UIKit, you do not need to also import Foundation. UIKit is a collection of standard APIs specific to the iOS interface. It includes views like UITableView and controllers like UIViewController. You will need to important this whenever working on the UI of iOS apps.

Cocoa Touch

This template is useful for subclassing classes from UIKit and potentially having Xcode generate interface builder files for you. If you don‘t need either of those things, you should just use the Swift Class template. Only have your object inherit from NSObject if you really have to (if you need to be able to use your Swift class from Objective-C). If you don‘t inherit from NSObject, there are extra optimizations that the compiler can make and it avoids some of the slower parts of the Objective-C runtime.