Similarly to CocoaPods, Carthage is a tool which is used to manage external dependencies. But its idea is not to modify the project file and add all dependencies, neither is it here to support a central place where all dependencies are listed. Don't panic. Carthage does the heavy lifting for you. Namely, it downloads the source code and compiles a framework, which should be manually added to the project.
Again, you have a file where all dependencies are described. Carthage uses that file to fetch all dependencies one by one and builds those. Then, it's up to you if you want to include the libraries in your project. When you update the dependency descriptor file, a new version will be fetched and built, but you have to manually update the linked framework. The update could be automated though; you just have to link the output file and once a new version is built...