How do I open an .odt file? I have received an .odt file and I wanted to know if it is possible to open it on my S3?  I need to be able to copy, paste, edit and view comments.

OS: 4.1.1 
Galaxy S3
.odt stands for "OpenDocument Text", a format primarily used with OpenOffice and its derivates (StarOffice, LibreOffice, NeoOffice), but also supported (at least via filters) by other office suits, even Microsoft's. There are several viewers available for Android, but unfortunately no (free or cheap) editors – though the LibreOffice team is working on an Android version for over a year already.

You will find a list of Office suites for Android in my German Overview Office-Pakete und Text-Editoren (after all, for the list you don't need to know the language). I also marked those (partly) supporting OpenDocument formats, watch out for the keywords "OpenOffice", "LibreOffice", and "OpenDocument".

I have not tested them, but it seems there is one supporting reading and writing of OpenDocument formats: Office 2012: TextMaker Mobile (for texts), with separate apps for Calc and Presentation. Each of the apps is about USD 9, so they don't come cheap (compared to other Android apps). Coffice only supports reading those documents, but comes for free, same applies to OpenDocument Reader.