There are fads in fashion, music, as well as in video games and few flashes in the pan shined as brightly, or burned out as quickly, as the peripheral-based music game crazed that shook console gaming in the late 2000s.
Games such as Guitar Hero and Rock Band were cultural milestones for their time, spawning a legion of clones and even an inspiring an entire episode of South Park (spoiler: their assessment of these types of games was less than positive).
The main feature that ties these two games together is that both are ostensibly timed button press games combined with peripherals that simulate some kind of musical instrument, whether it be a guitar, keyboard, and, later, even microphones.
Guitar Hero “Guitars”
Another draw of both of these games is that each would boast of massive libraries of classics and even a few obscure tracks for players to listen to while they jammed out on imaginary instruments. Not only did this stellar soundtrack help solidify each game as a go-to party gaming experience but also broadened the audience for the game beyond that of traditional video gamers, bringing in people of all ages. The Guitar Hero peripheral-based music games also preceded the motion-control craze swept in on the industry by the advent of the Nintendo Wii so you could make a tenuous argument that games like Rock  and and Guitar Hero helped pave the way for that revolution.
Guitar Hero On-Screen Visuals
As each series matured they began to differentiate themselves through different themes and celebrity endorsements of their respective products. The core gameplay between each remains similar.
In terms of differentiation the most obvious difference between Guitar Hero and Rock Band is in the instruments on which they focus. Guitar Hero largely started out as a guitar-focused game with other instruments added later.