Items are the basic units or building blocks of an Omeka site. They can be many different things, like a photograph, a website, a book, an oral history, a spreadsheet, or an artifact.
Each item exists in only a single collection, but you can change collection associations for an item at any time.
Each item should have descriptive metadata.
Individual items may contain multiple files.
In order to build a collection, you first must have items uploaded and described on your Omeka site. You can organize your collections in whatever way works best for your site.
Viewing Omeka's Managing Collections Tutorial is a good way to begin.