Both are from one language-families. Both Ukrainian and Russian are very beautiful. There is approximatily 60 percent of words that are similar in this languages. Russian and ukrainians can understand each other but not entire speech because there are differences. Four letters - і,є,ї, and also "g" that I have not in the shift, are not in russian. And this four russian letters - ы,ъ,ё, э are not in ukrainian. There are differences in pronunciation also.There are many words borrowed from polish and german in ukrainian. Numbers are almost similar. Just pronunciation is different among numbers. There is also third language of this family - bielorusian, and it is very similar to ukrainian. More than 3/4 of words are similar.

I don't know exactly but I can say that I don't understand Ukranian at all (my mother language is Russian)

The mentality is different.