Where teams work together at one task, e.g. the same document or code, the piece of software used must be able handling that. Use this tag if that's a requirement to the software you're looking for.
Collaboration is working with each other to do a task and to achieve shared goals. This could be a document written by multiple authors simultaneously, or a software project with multiple developers involved. As multiple people will apply changes at the same time, these changes need to be coordinated – which is a very specific requirement for the software being used.
- translating (localizing) a software by those using it, see: Any collaborative tool/website to localize an Android app?
- team-working on documents, see: OS X client for Google Docs/Drive, incorporating the real-time collaboration feature?
- version-control: mostly used in development, when not only "merging changes" is important, but one also needs to access "older versions" and compare changes afterwards (e.g. to see where a bug might have been introduced). Can of course be used for other works (documents etc.) as well.