Well back in the 1970s he was probably like Lady Gaga is to us today. He's English not American but very popular in America. Elton had his start in blues in the 1960s, he was a trained piano player, he collaborated with Bernie Taupin to write some classic songs, his first big hit was 'Your Song'. (Elton mainly composed the melodies, Bernie the lyrics.) They had a great album, Yellow Brick Road.
Elton was known for his flamboyant style, his crazy glasses and costumes - though a lot of dudes were doing that at the time, with the glam movement. You can tell he has been classically trained on keyboards but there's also an unmistakeable blues influence.
His distinctive vibrato vocals also made him stand out, though they weren't very accepted when he first started out. I think Elton does great ballads - some of my favourites are 'Goodbye Yellow Brick Road', 'Skyline Pigeon' (though it's not very well known, only Elton could sing a song about a pigeon and make it cool), and 'The Last Song'. I didn't know until recently that 'Daniel' was actually a song about a Vietnam veteran.
He's had several decades of consistently high quality work, with the exception of when he was struggling with personal problems and drugs.
Elton and David Bowie were two of the very few English glam artists who managed to also make it in America during the 1970s. Elton had songs that charted in the American black music charts.