You're reading the documentation for an older, but still supported, version of ROS 2. For information on the latest version, please have a look at Galactic.
Glossary of terms used throughout this documentation:
An API, or Application Programming Interface, is an interface that is provided by an “application”, which in this case is usually a shared library or other language appropriate shared resource. APIs are made up of files that define a contract between the software using the interface and the software providing the interface. These files typically manifest as header files in C and C++ and as Python files in Python. In either case it is important that APIs are grouped and described in documentation and that they are declared as either public or private. Public interfaces are subject to change rules and changes to the public interfaces prompt a new version number of the software that provides them.
A client library is an API that provides access to the ROS graph using primitive middleware concepts like Topics, Services, and Actions.
A single unit of software, including source code, build system files, documentation, tests, and other associated resources.
ROS Enhancement Proposal. A document that describes an enhancement, standardization, or convention for the ROS community. The associated REP approval process allows the community to iterate on a proposal until some consensus has been made, at which point it can be ratified and implemented, at which point it becomes documentation. All REPs are viewable from the REP index.
Version Control System, such as CVS, SVN, git, mercurial, etc…
The C++ specific Client Library for ROS. This includes any middleware related APIs as well as the related message generation of C++ data structures based on interface definitions like Messages, Services, and Actions.
A collection of packages usually managed using a VCS like git or mercurial and usually hosted on a site like GitHub or BitBucket. In the context of this document repositories usually contain one or more packages of one type or another.