Ukrainian and Russian share about 60% of vocabulary, they have similar but not the same grammar patterns. They're related languages. However, Ukrainian has more in common with Polish than with Russian. Maybe they are like German, Norwegian and Dutch or English and French: you can understand something but not everything. Ukrainians do understand 99% of Russian because of the historical situation but not all of them (especially, in the west of the country) speak Russian.

Different vocabularies and alphabet, the grammer might be slightly different I suppose. As a native Russian I understand about a half or less of what they are saying and writing.

@K P Thanks for answers. I get a clear view of the problem now

And yes, it is true that Polish and Ukrainian share many words and not only words.

Reading an Ukrainian newspaper for Russian is not a problem. At first you are perplexed with many unusual words... A hour or a few later you read it fluently - still coming across a new word once in a while. The other story is that Russians do not try it:( I think, I have spent several hours reading Ukrainian texts overall. There are some that I read effortlessly and understand everything. I think, they are written by Russian speaking Ukrainians, who learned Ukrainian as adults. Some are tough (folk tales especially! When they call a domestic animal with a different word, you can't guess it from context!) The opposite with Bulgarian: At first Russian is surprised with so many familiar words.... But... In other words Bulgarian seems easy at first but needs MUCH more time to learn. It is NOT honest to compare languages by newspaper. Language of TV and newspaper is simple. Speech is a different story. Ukrainins form Kiev have difficulty understanding speakers of Western Ukrainian dialects themselves! Once I watched a football game with Ukrainian commenters: I understood almost everything that one of commenters said, but I couldn't understand most of what the other was saying.