If you've ever played the vanilla version of mount and blade warband then you will probably know what to expect from viking conquest reforged. Essentially it's an expansion that blows it out of the water, adding in a lot more features that make it a much more complex and immersive experience.
For example, the world is a living thing that changes over time, with historical and random events as well as your own actions all contributing to how it evolves. The world is also much more detailed than before with over 300 new cities, castles and villages to explore as well as a huge number of extra NPCs. Including legendary figures like Ivar the Boneless and Sigurd Snake-in-the-Eye, royalty like Ragnarssons and Haraldssons as well as a whole bunch of other Jarls, Lords and Kings.
In addition, there's a massive range of additional gameplay options to enjoy such as a more complex religion system featuring two faiths (Norse and Christianity) with monasteries, special NPCs and parties. A more detailed siege warfare system with options to reduce a defender by hunger or morale and even direct assault. Plus the ability to 'work' and gain an honest day's wage by committing yourself to a quarry, mine or farmstead.
Visually it looks far better than the vanilla warband as well with more realistically textured areas and improved water graphics. It's not next generation graphical but it's still pretty good and I can guarantee you'll be impressed by the jump it makes over vanilla.