Google Docs often refers to a suite of office applications that integrates well with Google Classroom and Google Drive. It is free and works well with mobile devices. At a minimum, we'd ask that students have access to Google Classroom, Google Drive and Google Docs.
Google Docs is a simple word processing application (similar to MS Word).
It allows collaboration between users (using the 'Share' button) and can be used to submit work in Google Classroom. Teachers can also give students feedback directly in the document.
Google Slides is a simple presentation application (similar to MS Powerpoint).
Students will likely use this app when viewing lesson resources shared by teachers, or if creating a presentation report to demonstrate understanding. Again users can collaborate on a piece of work.
Google Sheets is a simple spreadsheet application (similar to MS Excel).
Students will likely use this app when collecting data or when plotting graphs for reports.