Distinct from Alternative Country, Progressive County, Cowpunk, Psychobilly, No Depression, and Outlaw Country, all of which represent countrified rock or rockified country, the basic genre "country-rock" refers to the first wave of artists who blurred the dividing line between traditional country and contemporary rock styles. From 1968 to about 1975, country-rock was one of the biggest selling genres in rock, with fully mainstream artists like The Eagles, Linda Ronstadt, and Neil Young working in the idiom.
At its heart it was essentially country music, filtered through the rock experience; amplification, big backbeat, countercultural lyrical concerns, emphasis on hooks.
Rock 'n' roll, after all, was originally the marriage of blues and country; Muddy Waters plus Hank Williams, so to speak. By the time of the British Invasion, rock and country had gone their separate ways. Country audiences didn't dig the Beatles' long hair and British accents, and rock fans didn't dig country's corniness and redneck machismo.
The rift was a cultural one, more than a musical one. The styles remained more similar than dissimilar; even the British bands like the Beatles played some country music on their albums.
However, the true forefather of the genre is Gram Parsons, who died in 1973 at the age of 26, but managed in his short, tragic life to ignite what would turn into a multimillion dollar rock subgenre, and also bridge the gulf between the two styles, which were cousins after all.
Parsons, with his International Submarine Band, recorded what most consider to be the very first country-rock record ever, Safe At Home in 1967. The album went nowhere, but gained notice among musicians in particular, who were impressed by the convincing country music played by this rock group. In particular, 20-year old Floridian and Harvard theology major dropout Parsons seemed visionary beyond his years.
At about this time, the Byrds were undergoing a major lineup reshuffle. Up to this point identified as kings of the folk-rock movement, the Byrds invited Parsons to join. This shifted their sound dramatically from folk-rock to country, and their 1968 album with Parsons, Sweetheart of the Rodeo, was considered the first true country-rock album to receive mainstream attention. The country artists they drew from stylistically mainly were from the Bakersfield school; Merle Haggard, Buck Owens. The spirit was one of exploration.