From the developer's website:
Tidalis is a block-based puzzle game with casual appeal, hardcore depth, and an addictive new "streams" mechanic. The basic rules of the game are this: blocks fall down into the board and have a color and an arrow direction. If a stack of blocks exceeds the height of the board, you lose. In order to clear blocks, you must right-click and drag paths through the arrows to set up chain reactions of like-colored blocks.
If this sounds simple, that's because it is -- you'll be lining up lengthy chains within minutes. But you'll be surprised how much brainpower it takes to set up combos of multiple chains, and the many brainteaser-style puzzles include some real stumpers. Tidalis has co-op and competitive multiplayer modes (both online and offline); action-oriented modes and timer-less brainteasers; a lengthy, casual-friendly adventure mode; twenty unique game modes providing innumerable twists to the basic gameplay; dozens of special blocks and items; and over fifty minutes of beautiful music to go with the painterly art.