Each level has a different layout and level of difficulty. The groves that the balls travel along are more or less winding depending on the level that you're playing. Their length also shortens as the levels get more difficult.
Don't let the balls get to the end of the labyrinth, you'll need to use a sort of extractor tool to take a ball out of its place in line and insert it into a new spot.
To successfully complete a level, you need to get rid of the line of balls by grouping them together into threes. When three balls are grouped together, they disappear, bringing the rest of the balls further from the labyrinth's end in the process.