They were originally a three-man vocal group of the early
rock and roll era - O'Kelly, Rudolph, and lead singer Ronnie, eldest to youngest - with hits like "Shout" and "Twist and Shout", the latter of which was repopularized by
The Beatles, but the original is still better, except for the lame horn break.
They later had a brief stay at Motown Records in the mid-60's, with "This Old Heart of Mine" another addition to the canon of Isleys classix. Prior to this, they founded their own record label, T-Neck Records. One of those early T-Neck productions, "Testify", featured their lead guitarist at the time, Jimi Hendrix; I'm not sure if there are other recordings with him.
In the late 60's/early 70's, they, along with the likes of Isaac Hayes, Stevie Wonder and Marvin Gaye, pushed the envelope of soul music - e.g. "It's Your Thing", used, years later, by Burger King in some ads. They also started doing renditions of rock material, like Stephen Stills' "Love the One You're With" and Bob Dylan's "Lay Lady Lay", but they were neither as good as the originals, nor were they as good as the original Isleys songs by the subsequent 3+3 band.