difference between a "real" version and a snapshot version

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The difference between a "real" version and a snapshot version is that snapshots might get updates. That means that downloading 1.0-SNAPSHOT today might give a different file than downloading it yesterday or tomorrow.
When you build an application, Maven will search for dependencies in the local repository. If a stable version is not found there, it will search the remote repositories (defined in settings.xml or pom.xml) to retrieve this dependency. Then, it will copy it into the local repository, to make it available for the next builds.
For example, a foo-1.0.jar library is considered as a stable version, and if Maven finds it in the local repository, it will use this one for the current build.
Now, if you need a foo-1.0-SNAPSHOT.jar library, Maven will know that this version is not stable and is subject to changes. That's why Maven will try to find a newer version in the remote repositories, even if a version of this library is found on the local repository. However, this check is made only once per day. That means that if you have a foo-1.0-20110506.110000-1.jar(i.e. this library has been generated on 2011/05/06 at 11:00:00) in your local repository, and if you run the Maven build again the same day, Maven will not check the repositories for a newer version.
Maven provides you a way to can change this update policy in your repository definition:

    foo-repository
... true XXX
where XXX can be:
  • always: Maven will check for a newer version on every build;
  • daily, the default value;
  • interval:XXX: an interval in minutes (XXX)
  • never: Maven will never try to retrieve another version. It will do that only if it doesn't exist locally. With the configuration, SNAPSHOT version will be handled as the stable libraries.

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