"Ruthenia" is a latinized version of "Rus'," which is the root word for "Russia." The chemical element "Ruthenium," for example, is named after Russia.

In practice, however, the word "Ruthenia" came to apply only to lands outside the Romanov Empire. I suspect the ethnonym "Ruthenian" enjoyed more popularity than the placename: it was the German / Habsburg word for people that today are called "Ukrainians." If you look at 19th century Encyclopedias published in German, they refer to Ruthenians; the ethnonym "Ukrainians" doesn't become widespread in German reference works until the twentieth century. At the time, scholars usually saw the Ukrainians as "little Russians," i.e. as a variety of Russians, but German scholars distinguished "Russen" (Russians) in the Romanov Empire from "Ruthenen" (Ruthenians) in the Habsburg lands.

In the twentieth century, a different meaning has become popular. There's also a bit of Ukraine that used to be part of Czechoslovakia, and before that part of Hungary: the district around Uzhhorod. That territory has lots of different names. Two names are "Subcarpathian Rus" or "Transcarpathian Rus" (depending if you are looking at the region from Prague or Kyiv, it's either "on this side of the Carpathians" or "on the far side of the Carpathians"). Another name for this region is "Ruthenia."

There's also a dispute as to whether the Slavs who live in the Uzhhorod district ("Ruthenia") are Ukrainians or a separate people. Those who believe they're a separate people tend to call them "Rusyns." The minority in neighboring Slovakia, and members of that minority are also described as Rusyns. There's a Canadian scholar of Rusyn origin who studies this part of the world named Paul Robert Magocsi. Magocsi argues for Rusyn distinctiveness, so look for his work if you want further information.

Interesting question, because I had the same question after reading about Roxelana wife of Sultan Suleiman the Magnicent.

"the word Roxolania was used to refer to the province of Ruthenia (or Rutenia ) in the Western Ukraine, which was at different times known under the names of Red Rus , Galicia , or Podolia (that is, eastern Podolia that was under Polish control at the time), while present-day Russia was called Muscovy, or Muscovy Rus , or the Duchy of Muscovy. In antiquity, the word Roxolani denoted both a nomadic Sarmatian tribe and a settlement on the Dniester River (presently in the Odessa region in the Ukraine"

Source: Roxolana “The Greatest Empresse of the East” by Galina Yermolenko

They are basically East Slavic people from Western Ukraine with distinct culture and traditions. Today they call themselves Rusyn(ethnic) or Ukrainians(ethnic or Citizen), their historical name was Ruthenian.