In Spots you must form lines(vertical, horizontal, diagonal) with the spots on the board. Lines are formed by aligning spots of the same color. The minimum spots you must align to form a line depends on the configuration. You may move a spot from one place to another only if there is a path between the two places. You must score as high as you can before the board fills up with spots.
The difference between spots and other games like it is that Spots is highly configurable. Unlike others that give you a predefined game type, in Spots you can change the grid size, the total number of different spot colors, the minimum Spot Line etc. resulting in approximately 7000 combinations of games. So you'll never get bored playing the same game type ever and ever again.