Explain MVVM Architecture with example

suppose you have to develop a desktop app , and you are using WPF,

obviously you can write functionalities (UI related, not business related) directly in code behind but what if you want to reuse those functionality in anyother application , the form and form.cs (code behind) are so tightly coupled that can not take benefit of reusabilty.

now suppose all your functionalities are in another class and you can easily bind them to any of other UI, wouldn't it be awesome.

That is why we use viewmodel concept. 

now your xaml will be bind to viewmodel, the same viewmodel can be bind with multiple Xaml.

you can not bind button click event of one page with other but if you use viewmodel then the same  command can be reused with other button's in any of your form (Xaml).