Open A is an alternate guitar tuning in which the strings are tuned to E2, A2, E3, A3, C#4, E4 (low to high). Starting from the lowest note, the interval structure corresponds to 5, 1, 5, 1, 3, 5. So in open A tuning an A major chord sounds without the need of fingering. Note: The root A is on 5th string, not on the lowest 6th string as in most other tunings.
The use of open A (and its lower tuned twin open G) is very popular among blues singers playing a slide guitar. Not only artists like Muddy Waters and Robert Johnson spring to mind here, but also the White Stripes' Jack White who used it to great effect in the seminal hit record "Seven Nation Army".