Spreadsheet document files are the subset of ODF files.
Spreadsheet files have .ods file extensions.
The content (sheets) is stored in content.xml subdocument.
Picture 1: content.xml subdocument

As we can see on Picture 1, sheets are stored as xml
elements. They contain column and row definitions, rows contain cells and so
on… On picture is data from one specific document, but from this we can see
basic structure of content.xml file (you can also
download full ODF
specification).