Illustrator comes with many patterns that you can access in the Swatches panel and in the Illustrator Extras folder on the Illustrator CD. You can customize existing patterns and design patterns from scratch with any of the Illustrator tools. Patterns intended for filling objects (fill patterns) differ in design and tiling from patterns intended to be applied to a path with the Brushes panel (brush patterns). For best results, use fill patterns to fill objects and brush patterns to outline objects.
When designing patterns, it helps to understand how Adobe Illustrator tiles patterns: