The Beatles' 19th single in Britain—"Get Back," backed with "Don't Let Me Down"—was released April 11, 1969, so the song was already well known when the Let It Be album was released a year later.
However, the single version (available on Past Masters) was recorded January 28, 1969 (as was "Don't Let Me Down"), while the album version was recorded the previous day—and it shows. The single version is more powerful, and the band attacks it with a bit more confidence, perhaps the result of giving the tune an extra night to sink in.
Other differences are the result of mixing; the single version (3:11), which benefits from a healthy dose of reverb, starts off cold and features nothing but pure Beatles-style R&B, fading out after some extended jamming and riffing by Paul McCartney ("Your mama's waiting for ya, wearing her high-heel shoes and her low-neck sweater").
The album version opens with some January 27 studio chatter (including John Lennon's "Sweet Loretta Fart, she thought she was a cleaner, but she was a frying pan" parody) and ends with a snippet of the January 30 rooftop performance, with Lennon hoping the band had "passed the audition."
Both versions feature Lennon playing lead guitar, getting a pleasing P90 tone out of his sanded-down Epiphone Casino; George Harrison on his custom-built rosewood Telecaster and McCartney on a Hofner 500/1, most likely his 1963 model. Both versions also feature brilliant playing by longtime Beatle friend Billy Preston, a recent Apple Records signing and true professional who melded quickly with his new, albeit temporary, band mates.
The song's chorus was initially inspired by the plight of Kenyan Asian refugees who were bound for Britain; the phrase "get back" wasn't meant in earnest, however; it was McCartney's satirical comment on the resulting racism brewing in Britain.