gh-109414: Add some basic information about venvs in the introduction.

Doc/library/venv.rst

@@ -30,6 +30,23 @@ When used from within a virtual environment, common installation tools such as
 `pip`_ will install Python packages into a virtual environment
 without needing to be told to do so explicitly.
 
+A virtual environment is:
+
+* Used to contain a specific Python interpreter and software libraries and
+  binaries which are needed to support a project (library or application).
+
+* Contained in a directory, conventionally either named ``venv`` or ``.venv`` in
+  the project directory, or under a container directory for lots of virtual
+  environments, such as ``~/.virtualenvs``.
+
+* Not checked into source control systems such as Git, Mercurial etc.
+
+* Considered as disposable -- it should be simple to delete and recreate it from
+  scratch. You don't place any project code in the environment
+
+* Not considered as movable or copyable -- you just create a new environment in
+  the target location.
+
 See :pep:`405` for more background on Python virtual environments.
 
 .. seealso::