Pointillism is a technique of painting in which a lot of tiny dots are combined to form a picture. The reason for doing pointillism instead of a picture with physical mixing is that, supposedly, physically mixing colors dulls them. Most of the painters of Seurat's time blended the colors to make a picture with a smoother feeling than Seurat's bright, dotty works.
When two colors are right next to each other your eye mixes them in a process called, "optical mixing." Using optical mixing rather than physical mixing can create a brighter picture. Painting a pointillist piece is a slow and painstaking process. Seurat's famous "A Sunday in the Park on the Island of La Grande Jatte" (more commonly known as "Sunday in the Park"), which covered a wall (81 inches by 120 inches), took him two years to complete. He was known for amazing devotion and concentration. The dots in a pointillist painting can be as small as 1/16 of an inch in diameter! Based on these measurements, "Sunday in the Park" has approximately 3,456,000 dots!
A Sunday in the Park on the Island of La Grande Jatte