Ukrainian is one of the names of one of (East Slavic) languages of Rus' (Rusian) and core literary language norms formed on the basis of the old ancient Principality of (Kyivan) Rus.
There is evidence of human presence in Ukraine dating back several millennia. The people who lived on the lands of modern Ukrainian in different epochs was known under such names as Scythes, Sarmatians, Goths, Huns, Slavians, Russians, Tartars, Cossacks and finally - Ukrainians.
The history of literature, literary language and alphabet can be traced back to the translations of Christian texts (among them - Psalter and Holy Scripture) in the IX-XIIth centuries, which formed the basis of language norm in the Principality of Rus' with the center in Kyiv.
Kyiv, the modern capital and largest city of Ukraine, seems to have been established in the 5th century AD by the Polanian tribe - one of Slavic tribes who inhabited Ukrainian steppes ('pole' - POHleh), as a Slavic trading post. Hegemony over the city and the surrounding area shifted between different groups of Slavic and non-Slavic peoples, and it eventually grew to become the capital of Rus'. The early Glagolitic and Cyrillic writing systems were developed in other Slavic regions at this time.
Later, in the XIII century, the lands of Principality of Rus' and it's capital was pillaged by the Great Horde, an event which deepened the already evolving split of the varied and loosely-connected one Rusian (Ruthenian) society into different parts, forming the modern Ukrainian (Rus'-Ukrainian), Belarusian (White Ruthenian) and Russian (Great, or extended-wider Rusian [Russian]) nations with related, but distinct languages.
In XIII-XIV cc. territories including Kyiv and areas west and south of Kyiv) formed the first uniquely Ukrainian state, Halych-Volynia, and other principalities, which were finally conquered by Poland and Lithuania in the 14th-16th centuries. The non-Rusian (Ruthenian-Ukrainian) territories to the east and north-east were left under the rule of princes of ancient Rus' dynasty, the Rurik descendents, under the determinative Mongolian suzerainity.
During the epoch of XIII-XVI cc., the language in Ukraine gradually incorporated new words from the languages of surrounding peoples and countries, among others - Greek, Latin, Pole, Turkish, Turko-Tatar and so on. Also in this period people of this area further developed a second name to designate Rus' 'propria' - "Ukrayina" - Ukraine. The evolving Ukrainian language became further differentiated from the nascent Russian language, which incorporated Finno-Ugric and Mongolian words; and later form the Belarusian language, which incorporated Lithuanian influences.
In XIV-XV сс. areas of Ukraine began to free themselves from dominance of Great Horde. Some of them fell under Polish and Lithuanian rule, but others (self-governing Cossack communities of the steppe) did not; they strove, instead for independence. In XVII century, central Ukraine, (Rus'-propria (menora)) gave birth to the Cossack State. After regaining independence, the revived Ukraine-Rusian Cossack State was incorporated into the structure of Russian (Muscovite)czardom and society. In XVIII century, a lot of the Rusian-Ukrainian citizens were enslaved as Muscovite (Russian) bondmen. In XIX century, their descendants regained partial indepence as individuums, but had their language declared non-existant; so, in 1917, they proclaimed the revival of a free, independent Republic of Ukrainian-Ruthenian (Rusian) Cossack nation, with Ukrainian as the main official language, with secondary roles for the Russian, Polish and Yiddish languages. Later, the Republic was conquered to become part of USSR until 1991.
So the Ukrainian language is a direct descendant of Proto-Slavic, from which all the Slavic languages evolved, being separated after the Migration Period in between the 4th and 7th century. The evolution of Ukrainian to its contemporary stage began in the 12th century, and was completed during the 14th and 16th centuries, with perduring transitional and regional forms. The old names of the Ukrainian language were Rus', Ruthenian or Rusian language, that should not be wrongly associated with contemporary Russian, which, in modern English, is better translated as "Great Russian."
The Ukrainian language is less influenced by Old Church Slavonic, in comparison to Russian. Also some Latin, German, Polish and Turkic words were loaned into Ukrainian, as well as Ukrainian words into Russian, Polish, Hungarian, Tatar, Romanian etc.