Subversion, often abbreviated as SVN, is not the only or best version control software available, but since it is used by WordPress to manage user-contributed plugins we have to talk about it specifically. However, we aren't bowing in blind obeisance here: SVN has earned its place as one of the standard version control applications because it is reliable, it has a good set of features, and it is relatively easy to use.
Before we get into details on how to accomplish specific tasks, let's take a moment to discuss some of the terms and concepts so you can see the forest for the trees.
The first concept to introduce is that of the "repository", or the repo. An SVN repository holds all of a project's files and folders and all of its incremental revisions. You can set up a repository anywhere, but it is commonly set up on a remote server separate from your development environment. At any time, you can download the latest version of your...