What is ODS?
LibreOffice and OpenOffice's spreadsheet, and the ISO-standard one - the same ODF packaging an ODT uses, with sheets where a document has pages. Every cell stores its value and, separately, the text shown for it, which is why a figure formatted as currency can still be read back as the number underneath.
What is XLSX?
A modern Excel workbook, which is really a zip full of XML. Unlike a CSV it knows what a number is, holds multiple sheets, and keeps formulas and formatting. It's what people mean when they say 'send me a spreadsheet'.