One of the first things that I learnt when I took a survey course in Russian history was that there are two words for Russian: русский and российский (russkii and rossiiskii). The Russian language and other elements that might be termed ethnically Russian are russkii, whereas things that relate to the Russian state (i.e. the Russian Federation) are rossiiskii, and a Russian citizen is therefore a rossiianin or rossiianka. This russkii / rossiiskii distinction may come across as too neat, especially historically, but it is nevertheless a useful tool.
The Russian Federation is a multi-ethnic, multi-religious state, where diverse peoples live. Indeed, the preamble of the constitution starts: Мы, многонациональный народ Российской Федерации, ... (We, the multinational people of the Russian Federation, ...). The use of the singular for народ (narod), a people, but also a word that is difficult to render adequately in English, is quite interesting and telling. There is a single Russian people (defined by citizenship), but composed of many different nationalities, among which are Russians. The preceding sentence hardly makes sense in English, because there is only one word for both russkii and rossiiskii, though perhaps it might help to draw on the differences between the concept of British citizenship and the different peoples, cultures, and languages that exist in the UK.
Many citizens of the Russian Federation are not ethnically Russian, by language, tradition, and culture. Perhaps another way to put it is thus: many rossiiskii-Russians are not russkii-Russians. Equally, there are citizens of the Russian Federation (rossiiskii-Russians) living outside the country, and there are ethnic Russians (russkii-Russians) who are not citizens of and do not live in the Russian Federation. The ethnic mix in what was the former Soviet Union could have been explosive, especially in view of what transpired in Yugoslavia after the fall of communism. Given many instances of deliberate starvations and deportations during the Stalin era, it is actually surprising that with the exception of the Caucasus region the Russian Federation has remained largely a stable state.
The Russian Federation has been giving out passports to the Abkhazians and South Ossetians, many of whom are not ethnic Russians, following the largely unrecognized independence of Abkhazia and South Ossetia, which would justify in future Russian intervention in these areas, on the grounds of protecting its citizens. In other words, protection for the rossiiskii-Russians.
In the current situation in Crimea and Ukraine, where there are some rossiiskii-Russians, President Putin seems to be attempting to justify the actions of the Russian Federation in protecting the interests of the russkii-Russians living in Ukraine. Naturally this would worry other states with large populations of Russian speakers, since the Russian Federation may in the future use the same argument to intervene elsewhere. And does the principle operate one way? If the Russian Federation can claim a right or responsibility to protect ethnic Russians, cannot other states claim the same right and responsibility to protect minorities in the Russian Federation?
Identity, nationality, ethnicity, citizenship, and statehood are intricately and delicately linked, not only in Russia, but in many places across the world. Political moves and narratives can disrupt that unstable balance, even if it can be expedient to do so to achieve a political or military goal. What will be the consequences for Ukraine and Russia and their peoples, whatever happens in Crimea?