Two-dimensional cel animation and stop-motion animation both fall under the category of traditional animation, even if both may use digital methods of filming in the end. What matters is the method of producing the animation itself; cel animation generally involves hand-drawing, hand-inking, and hand-painting each frame on physical paper and cels, while stop-motion involves working with physical models and objects captured on camera one frame at a time.
Computer animation can be either two-dimensional or three-dimensional. Two-dimensional computer animation often involves a virtualization of the traditional two-dimensional animation workspace, bringing pen and paper into the digital environment to recreate cartoon animation workflows and styles. Three-dimensional computer animation tends to involve a hybrid of workflows following traditional timelines adapted to working in a virtual three-dimensional space. Either way, if you're animating on-screen you're working with computer animation.
Pixar's animation style is three-dimensional computer animation. It involves the use of a computers to animate within a three-dimensional environment.