Introduction + Basic details
Windows 10 Mobile is a mobile operating system developed by Microsoft. It is an iteration of the Windows Phone product line and a successor to Windows Phone 8.1, but is marketed as being an edition of Windows 10, Microsoft's operating system for personal computers, as part of Microsoft's plans to unify their platforms into one converged operating system.

Windows 10 Mobile aims to provide greater consistency with its counterpart for personal computers, including more extensive synchronization of content, a new universal application platform that allows one app to run on multiple Windows 10 devices such as PCs, mobile devices and Xbox, as well as the capability, on supported hardware, to connect devices to an external display and use a "PC-like" interface with mouse and keyboard input support. Microsoft has built tools for developers to easily port some iOS apps with minimal modifications. Windows Phone 8.1 smartphones are eligible for upgrade to Windows 10 Mobile, pursuant to manufacturer and carrier support. Some features may vary depending on hardware compatibility.

Windows 10 Mobile is designed for use on smartphones and small tablets under eight inches in screen size, running on ARM as well as IA-32 processor architectures. Windows 10 Mobile entered public beta for selected Lumia brand smartphones on February 12, 2015. The first Lumia smartphones powered by Windows 10 Mobile were released on November 20, 2015, while eligible Windows Phone devices are scheduled to begin receiving updates to Windows 10 Mobile in early 2016, pursuant to manufacturer and carrier support.

Some major apps available on Windows 10 Mobile
Google Search
Facebook
Twitter*
WhatsApp
Instagram* (In Beta release)
Facebook Messenger* (Windows 10 version in Beta release)
Netflix*
Pandora*
Dropbox*
Hulu*
LINE*
Amazon*
eBay*
Spotify
Shazam*
Skype*
...and more.

Some major games available on Windows 10 Mobile
Ashpale 8: Airborne*
Crossy Road*
Cut The Rope*
Cut The Rope 2*
Candy Crush Saga*
Minecraft
Fruit Ninja
Angry Birds*
Temple Run
Assassin' Creed Pirates
UNO & Friends*
Dead Trigger 2*
Gears of War: Ultimate Edition for Windows 10
Lara Craft GO*
Sonic Dash*
...and more.

*Universal Windows App
These apps can run in your PC, Phone and/or Xbox.

Latest release
Version 10.0.10586.122
Slow ring: 2 March 2016
Release Preview: 2 March 2016

Preview release
Version 10.0.10586.164 (1511)
Slow ring: 8 March 2016
Release Preview: 8 March 2016
Public Release: 8 March 2016

All Windows 10 Mobile related discussions should be posted here!
This is not the final version of the OP.

Sources: Wikipedia and Windows Central

Universal Windows Platform: Explained

Universal Windows Platform (UWP), is a platform-homogeneous application architecture created by Microsoft and first introduced in Windows 10. The purpose of this software platform is to help develop Metro-style apps that run on both Windows 10 and Windows 10 Mobile without the need to be re-written for each. It supports Windows application development using C++, C#, VB.NET, or XAML. The API is implemented in C++, and supported in C++, VB.NET, C#, and JavaScript. Designed as an extension to the Windows Runtime platform first introduced in Windows Server 2012 and Windows 8, the UWP allows developers to create applications that will potentially run on multiple types of devices.

Windows 10 Mobile is truly the most beautiful OS ever. I'm forever bound to this OS, and will never ever think of going Android or iOS despite our 1% market share. There is always time for everything, I believe Windows will make a huge comeback in a few years.

Woooooooooooooooooooooaaaahhhhhhhhhhhhhhhhhhh I'm sooo excited to update my Lumia to Windows 10 Mobile!

Originally posted by Coklek92 What should i do with my Lumia 920? Use it as hammer i guess

Originally posted by Legend E Yeah, it's not available on 920 Wait we're never gonna get it?