Microsoft's venerable Windows 7 operating system has crossed yet another threshold (no pun intended) on its way to eventual irrelevance. As of today, January 13, 2015, the operating system has exited what Microsoft calls mainstream support.

But, what exactly does that mean and is there reason for concern?

By entering the extended support phase, Windows 7 will no longer receive any new features or product tweaks. What's more, users won't be able to call Microsoft for free tech support regarding issues with the OS.

The good news, however, is that Windows 7 users will still be able to receive critical security updates for the foreseeable future. The extended support phase runs from now through January 14, 2020, so there's no need to fret about upgrading just yet if you are content with Windows 7.

Windows 7 arrived in late 2009 and continues to be a leader in terms of Windows market share to this very day. Microsoft sold more than 100 million licenses within just six months of its release, making it the fastest-selling Windows OS to date.

It was followed by Windows 8 three years later but given its radical new UI, many users have elected to stick with what they know in Windows 7 as it's generally considered to be more stable than its predecessor, Windows Vista.

With Windows 10 expected later this year, however, we might finally see more users say goodbye to their beloved Windows 7.