Brian was basically in charge of the group in the early days. A lot of people who saw him back then thought that he had even more charisma and prescence than Jagger. His role as group leader was diminished when Andrew Loog Oldham chose to focus more on Mick and Keith and their budding songwriting talents (a ploy to echo the success of Lennon/McCartney, but it did actually work) Brian, who is thought to have suffered from bipolar disorder, spiraled more into drinking and drugs (NOT HARD DRUGS!!! He was very afraid of them, shedding more light on his paranoid tendencies, so remember that before you go pinning him as some drugged-out after-school special) and became less productive in the studio, eventually rarely showing up. This is especially sad because in the earlier days, mostly 1963-1967, he contributed very, VERY innovative musical ideas.
The thing about him was that he was a guitarist along with Keith, but also a multi-instrumentalist. It was said that he could find and obscure instrument and make beautiful music out of it, and the Stones songs that he plays on certainly prove this to be true.
Still, he became more and more alienated from the group, and was busted for pot three times (however, like Mick, Keith, and Beatle George Harrison's arrests, the officer was corrupt so although of course they all did drugs, some evidence was fabricated) and when he couldn't get a visa to tour the US with the Stones, they unceremoniously kicked him out of the band he had formed. This was in June, and he died July 3rd, 1969 due to drowning in his swimming pool.
Many think it was suicide, but this is much debated. For starters, even though he was prone to violent mood swings, he was described by many as in good spirits around July 3rd. His death was ruled "misadventure," and the press reported it as an asthma attack. While he did suffer from asthma, there was water in his lungs- had he suffered from an attack, his windpipe would have closed up. Also, there was a builder around the house who acted very suspiciously. While the builder could have murdered him, it's also possible that Brian's damaged heart could have just given out, or that he could have had a seizure, or that one of his mood swings had turned fatal. No one really knows, which is the worst part. He was only 27, and three more significant musicians (Janis Joplin, Jimi Hendrix, Jim Morrison) died shortly after at the same age, so Brian Jones is unfortunately the founder of the 27 club.