Definitely, Russian and Ukrainian are different languages. Although, they have much similar. For example, the difference of alphabets is just several letters (i, ї, є, ', ґ), also some letters sound different from Russian (е, и etc.).

Grammar is also have as similarities, as difference. And vocabulary, of course.

Anyhow, those are the relative languages. So if you speak Russian, you will definitely understand what you were told in Ukrainian. And vice versa.