It's a replacement for Objective-C, but you can mix and match code in the same project.
You'll need to know both, at least for the time being - many open source libraries are written in Objective-C, so if you need to make tweaks, you'll need to understand Objective-C. Also, a lot of existing tutorials aren't written for Swift.
One really important thing to note is that even though the languages are different, the underlying iOS/OS X libraries and frameworks are the same. The hard part isn't really learning the syntax of the language, it's understanding how to accomplish what you want. For instance, showing pins on a map or displaying items in a collection view.
FWIW, I teach iOS app development, and I think that beginners should learn Objective-C, not just Swift.
You'll need to know both, at least for the time being - many open source libraries are written in Objective-C, so if you need to make tweaks, you'll need to understand Objective-C. Also, a lot of existing tutorials aren't written for Swift.
One really important thing to note is that even though the languages are different, the underlying iOS/OS X libraries and frameworks are the same. The hard part isn't really learning the syntax of the language, it's understanding how to accomplish what you want. For instance, showing pins on a map or displaying items in a collection view.
FWIW, I teach iOS app development, and I think that beginners should learn Objective-C, not just Swift.