The Sindar, also known as Grey Elves or Middle Elves, are the first Elves to settle in Beleriand. As they didn't go to Valinor, they could not be counted among the High Elves, therefore earning the title of Middle Elves. But their contact with the Valar and the High Elves did grant them a higher status than the Silvan Elves, who either stayed in Greenwood and Lorinand or arrived at Beleriand at a later time.
The Sindarin lived in the subterrenean realm of Menegroth in the woods of Doriath, and were ruled by the High-Elf Thingol. Luthien, daughter of Thingol, was therefore partially a Sinda.
After the sack of Doriath, several of its surviving denzins escaped east. Among these were Galadriel (a Noldo), Celeborn and Oropher. Galadriel and Celeborn would come to rule over the Silvan Elves of Lorinand, while Oropher would rule over those of Greenwood, styling himself as the "Elven king." He mimiced Thingol's practice by establishing an underground realm under the Mountains of Greenwood.
Oropher and his progeny, Thranduil and his own son Legolas, would remain distinct from their Silvan subjects through their blonde hair. Even while Thranduil was willing to adopt a Silvan orphan, Tauriel, he stated that he would not allow Legolas to pledge himself to her, given her lesser status.
It may be that the Sindar possesed, among other things, a higher tolerance to intoxication, being that Legolas won a drinking contest, while the Silvan Elves of his realm were heavily inebriated on account of drinking from Thranduil's wines.