Content items are instances of content types, just like objects are instances of classes. Content items are the heart and soul of Orchard: they store all the data you need to handle and display.
Content items are always versioned every time you modify any data on them and are never hard-deleted to keep the integrity of your database. From code they can be interacted with through the Content Manager service.
This page and many others here are part of the open-source, portable package of Orchard 1.x 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!
Note that the Library also has an Orchard Core version, check it out here!