The Content Manager is the important service in Orchard that you can use to interact with content items from code.
When developing a module you can use the Content Manager to create, update, retrieve and remove (soft delete) content items, fetch different versions of the same content item as well as import/export items, fetch items' metadata or build display and editor shapes. You can find a detailed example of how to use the Content Manager in the Training Demo module.
This page and many others here are part of the open source, portable package of Orchard goodies: the Orchard Dojo Library. The Library is freely accessible by anyone; just go to its repository to download its full content (you can even download the textual content of the Library as a big concatenated document in HTML), file issues or fork it! You can even edit it online on Bitbucket, just click on the Edit link in the upper right corner!