A Bug's Life is Disney's second completely computer animated film following Toy Story from a couple of years ago. Coming right at the heels of Antz, comparisons are inevitable. The primary difference is that Antz is essentially an animated Woody Allen film, and A Bug's Life is a glitzy Disney profit-making machine.
A Bug's Life is the story of Flik (Dan Foley), an ant, who much like the Z character in Antz, is a bit of an iconoclast. Flik gets into trouble when he ruins the harvest gathered for a band of vicious grasshoppers, led by Hopper (Kevin Spacey). Needless to say, Hopper isn't pleased when he finds out and threatens dire consequences if the ants don't make up for their loss by the time the last leaf falls. As the ants slave away to redo all their work, Flik heads off to find warrior insects who will fight to protect the ant hill in exchange for money. Flik runs into a troupe of circus performers and thinks they're warriors and brings them back, convincing the colony that they can protect them. There is great rejoicing, but just before the grasshoppers return, the colony learns of the performers' true nature. As is usually the case in Disney animated films, the ending is a happy one and Flik comes through and manages to defeat the grasshoppers.
The animation (by Pixar) is technically superior to what is seen in Antz but the characters themselves are represented in a simpler fashion, presumably because of the intended audience (they make for great stuffed animals). It's sad and amusing that the creativity process is completely and inexorably tied to the marketing, but perhaps I'm just being cynical. In general however, the quality of animation definitely makes this movie another milestone for Pixar.
The voices are done fairly well but I think A Bug's Life could've had better casting. I think the main character was also suited for Woody Allen's on-screen persona. Depending on your preference (and the age), you might want to go for the more neurotic Antz over the sugar-sweet A Bug's Life.