Symfony is a set of reusable PHP components and a framework.
Best way to get to know Symfony is to go through a quick tour - four chapters, each 10 minutes long. After that, reading the documentation is advised and is the best way to learn it thoroughly.
Symfony uses bundles for applications and to extend and reuse functionality in them. A large and vibrant community that has evolved around Symfony provides bundles for almost anything. They can be found at KpnBundles.
You can also get certified or meet the community at one of the Symfony events.
Useful resources and tutorials when working with Symfony: