A Swift package is a folder containing a manifest file and source files used to build software products.
The package manifest (a file named Package.swift at the top level of the package folder) defines the package’s name, products, targets, and dependencies on other packages. The manifest file is written in Swift using API from the Swift Package Manager's PackageDescription library. For more details on the manifest file, go to Package.
A package product defines the externally visible build artifact, such as libraries and executables, that are available to clients of a package. A package target defines a test or module from which the products in a package are built. Targets may have dependencies on targets in the same package and dependencies on products from its package dependencies.
A package dependency enables a package target, or Xcode project, to use a product in another package. A package dependency is specified by a URL to the remote Git repository containing the package, and the versions of the package that are supported by the client. The format of a package version uses the Semantic Versioning specification, which is typically a three period-separated integer, such as 2.1.4.
The source files for targets can be written in Swift, C/C++, Objective-C/C++, or assembler, and are located under the Sources folder in the package. Each target can either contain only Swift source code, or any combination of C, C++, Objective-C, Objective-C++, and assembler source code. The source files for Test targets are written using the XCTest framework, and are located under the Tests folder.