ODT stands for OpenDocument Text, the word processing format of the OpenDocument Format (ODF). It is the format LibreOffice Writer saves in by default, and it was designed from the start as an open standard: anyone may write a program that reads or writes it, without a licence and without depending on a single company.
Where it comes from
ODF grew out of the XML file format of OpenOffice.org, the open-source office suite Sun Microsystems released in 2000. The standards body OASIS took the format over and published OpenDocument 1.0 in 2005. A year later it became the international standard ISO/IEC 26300. Later versions followed: ODF 1.2 (approved by OASIS in 2011, adopted by ISO in 2015) and ODF 1.3 (2021).
Because it is an open, vendor-neutral standard, several governments have adopted ODF for exchanging documents. The UK government, for example, chose it in 2014 as its standard for sharing editable documents.
What is inside an ODT file
Like a Word DOCX file, an ODT file is a ZIP archive of XML files:
mimetypeis the first file in the archive, stored uncompressed, and contains just the format's MIME type. It lets a program identify the file without unpacking it.content.xmlholds the text, tables and their formatting.styles.xmldefines the paragraph and character styles and the page layout.meta.xmlstores the document's metadata: author, dates, word count.Pictures/holds embedded images, andMETA-INF/manifest.xmllists every part of the package.
The same standard covers spreadsheets (.ods), presentations (.odp) and drawings (.odg), all packaged the same way.
ODT and Microsoft Word
Microsoft Word opens and saves ODT files, and so do Google Docs and most other word processors. Plain documents convert cleanly. Elaborate layouts can change a little on the way between programs, because ODF and Office Open XML describe some features differently, and fonts that are not installed get replaced. If a document has to look exactly the same for every reader, send it as a PDF as well.
How the spell checker helps
Open an ODT file in the Spell Checker and its text is checked for spelling, grammar and punctuation, plus style suggestions in Picky mode. When you have applied the fixes, download it as ODT again: the document keeps its formatting (styles, tables and pictures), so it opens in LibreOffice just as it did before, only corrected. If the document is moving on to someone who uses Word, you can download it as DOCX instead. It is free, needs no account, and checks up to 150,000 characters at a time in more than 30 languages.
See also DOCX - Microsoft Word Document, the format ODT is most often exchanged with.