It was written and sung by George Harrison, who was learning the sitar from the esteemed Indian musician Ravi Shankar. The song features a sitar, a tabla, a tambourine and Tamboura Drone. Ringo Starr was the only other Beatle who played on this track (he was on the tambourine).
A few years later, the intro of the song was used in The Beatles animated film Yellow Submarine when George's character is first introduced.
"Yeah, it's Harrison's first true musical expirament into Indian culture and tradition, and thus is the precursor for other Indian-themed Beatles songs such as Within You, Without You and I, Me, Mine."
"Geez, How d'you know so much?"
"Urban Dictionary. Tells you all you need to know."