The "Young Turks" usually refers to either

A: The generation of middle and upper middle class ethnic Turks (often young military officers or intellectuals) around the turn of the century who were, broadly speaking, concerned with ideas of Turkish nationalism and a restoration of the Ottoman constitution of 1876. Ideologically this was all quite vague. They were not part of any unified group, but rather often formed a diverse array of local or regional nationalist societies, and it's clear that they often don't have any real ideological affiliation to the specific 1876 constitution but rather are in favor of it symbolically as an example of their commitment to constitutional rule.

OR

B. The Committee of Union and Progress (CUP) which was a specific organization composed largely with the class composition and ideological outlook I described above but who, in the years prior to WWI, actually seized power and ran the government. Their influence rose massively starting with the "Young Turk revolution" which they led, and was consolidated in a pair of elections, the second of which they won all but six seats in, a "counter-coup" in 1909 and finally in a coup d'etat in 1913.

As for "what did they do" well, the CUP from 1909 onwards was pretty much responsible for everything that happened in the Ottoman empire until 1918, including the pre-war policy of Turkification which contributed to Arab nationalist movements, the entry into the war on the German side, the Armenian genocide which I think most historians have put under the category of Bad Things. But they were also in favor of society-wide modernization and, to an extent, liberalization efforts many of which were carried on by Ataturk and are celebrated as part of his legacy. But given that "Young Turk" describes almost more of a turn of the century social milieu than any specific group or ideology, I think most historians, except when using it as a short hand or as a (admittedly quite cool) nickname, would probably prefer to use the CUP than "Young Turks" just to be as specific as possible. In other words, people are more likely to buy a book called "The Young Turks" than "The Committee of Union and Progress."

(also, C. it's entered the English language as an expression to apply to bright young ambitious upstarts in general, but I don't think that's what your question was referring to.)

Source wise: I'm afraid I'm not much of an Ottomanist but this is all covered as background in the first 30 pages or of Eugene Rogan's The Fall of the Ottomans and in more detail in Sean McMeekin's The Ottoman Endgame.