The SDF are heavily kurdish (and were originally based out of Kurdish-speaking areas), but they've basically absorbed most of the pro-democracy/secularist rebel forces. They basically have a ceasefire with the Syrian Army (though occasionally come to blows), and are currently mostly fighting against ISIL forces. The SDF are currently the ones working to take Tabqah Dam and liberate Thawrah and Raqqah, and will, by all chances, dominate northern and eastern Syria by the time ISIL is gone from that country. The USA heavily backs the SDF now, with equipment, training, air support, artillery support, and spec ops forces, as the SDF are, basically, the only good guys left in the civil war. The Russians also back the SDF, seeing them as a counter-balance to the Turks and FSA rebels. Assad's regime doesn't like them, but they would rather have a truce and negotiated peace than have to fight yet another enemy, especially when the SDF currently stand the best change of breaking the siege of Deir Ez Zur. The Turks are against them, as they see a the SDF as a way to establish a heavily Kurdish Autonomous Region, like Iraqi Kurdistan, on their border, thus threatening their own Kurdish-populated areas.