Bundles are containers of content items (pages, components, any items in Tridion)

Bundles is a new feature which came with Tridion 2013. Bundles enable grouping of content items (pages, components, schemas etc.) and enables the items to be published/unpublished, content ported together for a feature, go through the workflows together apart from many other possible use cases. Bundles act as the grouping container for the items.