The first question a newcomer to Subversion may ask to himself - What is this and how can I use it in my work? Speaking shortly Subversion can be characterized as a version control system which tracks the history of file and directory changes over time. All versioned files and directories reside as a tree structure (just like a general filesystem) in a Subversion repository. Both versioned files and directories can have versioned metadata - properties. Changes to such a tree are committed by clients to a repository in a single atomic transaction. Each commit operation creates a newer snapshot of the whole tree which includes all recent changes made in that commit operation as well as all previous unchanged data. Such a snapshot is called a revision. A Subversion repository starts its life from revision 0 where only the root directory exists. Then as files and directories are imported into a repository, changes to them are committed - new revisions are created, and each successful commit operation increases the revision number by one:
Subversion doesn't actually keep the whole contents in every revision. For every N-th revision it uses smart mechanisms to store only the differences that were made in that N-th revision itself (i.e. data differences between revisions N-1 and N). So, revisions allow you to retrieve any version of a file, or directory, or even of the whole tree (since a single revision is created for the whole repository tree) at any time, no changes will be lost. Each revision is a permanent snapshot of a tree, i.e. if an item is added to a Subversion repository it can not be removed from the repository entirely, because it can be always found in those revisions where it was added and changed. Although in this case item history is broken and it no longer exists in the repository starting with the revision where it's deleted.
A common scenario of using a Subversion repository is working with a copy of any subtree of a repository tree on a local computer and publishing results of this work into the repository. In other words, a client somehow changes files and directories taken from a repository and then commits his\her changes into the repository. Local versioned data tree is called Working Copy (WC).