Power metal is your stereotypical swords-and-sorcery fare. Lyrics dealt with the eternal struggle between good and evil, and album cover art evoked medieval or nordic imagery.
You could say “speed” is best used when describing songs, not artists. Thus, speed metal becomes a subset of power metal, and is characterized by double-time beats, virtuosic guitars, and high, clean vocals. As metal progresses, we will see vocal styles grow increasingly rough; but power and speed vocalists prefer clarity over aggression. This “higher, faster, louder” approach to metal can be heard early on in Deep Purple’s “Highway Star,” then in the music of Judas Priest and Helloween.
Yngwie J. Malmsteen is the quintessential electric guitar virtuoso. Beginning with Ritchie Blackmore and Uli Jon Roth, there had been a trend of fusing elements of classical and metal, which Malmsteen does like no other. He even goes so far as to directly quote Bach and Paganini. Here is something from his Rising Force album: