The Web is full of obituaries that were obviously prepared well before Newman's demise, and their format and the story that they tell--of Newman's distinguished acting career, his solid and long-lived marriage to the amazing Joanne Woodward, his enthusiasm for auto racing, his social activism, and his devotion to good works--ought to be familiar to anyone who has been semi-conscious for the past couple of decades.
However, if you want to understand precisely why Newman won the love and admiration of so many people, check out the remembrance written by Slate's always excellent Dahlia Lithwick. Lithwick worked as a counselor at one of Newman's Hole in the Wall Gang Camp in Connecticut for several years during the late 1980's and early 1990s. The camp, which is designed to enable children with cancer and other serious illnesses to forget about their health issues and focus on simply being kids, was the first charitable enterprise supported by Newman's Own profits.
Newman, who lived nearby, was a frequent visitor. To the kids, he was just "this friendly old guy who kept showing up at camp to take them fishing," and they "indulged [him] the way they'd have indulged a particularly friendly hospital blood technician." As it turns out, this is exactly what he wanted: