Simple enough, to progress through different levels you have to type in the falling words as fast as you can. The words fall faster as you go through higher levels. Longer words are also more prominent from levels 6 and up. Correct words fill up the green meter located on the right side of the game screen. Wrong words fill up the red meter. Gamelevel progress indicator is shown by a pile of papers slowly filling up at the corner.
This is the starting game interface screen.
You might notice several strips of words colored in black, yellow, blue, violet or red. Black is a score multiplier, yellow gives you the "slow" powerup, blue gives you the "ice" powerup, violet gives you the "wind" powerup and red gives you the "fire" powerup. The slow powerup slows down the dropping speed of the letters. Ice temporarily freezes the words on the screen, you will not lose any available powerups, although since they're frozen over, you won't be able to see the color anymore. You'll still be granted the powerup for that word. (Thank you David for the comment) The fire feature burns all the words on the screen. This is handy when the flurry of words become too overwhelming. The wind feature refreshes your red bar meter to zero. At the end of each level, your score is calculated and given a bonus if you've attained 100% and error free, meaning you've correctly typed each word. Different titles are available corresponding to your final score. You will be able to see your ranking among your friends who has the application installed.
Try to get all the words right during the early levels of the game. The extra bonus gained from this will really mean a lot in the end. Try to get all the special colored words first because this will prove very useful during tight spots. Hold back on using these special features unless you really really need to. Use the Ice and Fire feature when a lot of words are flooding your screen, so that you can start fresh. I also found it useful to type in slow (if you've got this extra) at once during the high levels before even the first word starts to fall. Take rests in between levels. All those falling letters can cause severe strain on the eyes, making it harder for you to focus and concentrate. And lastly, practice makes perfect.
Get bonuses if you get all the words right.