Atari's response to the Tetris mania in the early 90s. Originally a coin-op, afterwards released on just about every computer and console available at that time, from Amiga to Nintendo's GameBoy.

In Klax multicolored blocks are rolling down on a "conveyor belt". You must grab the blocks and arrange them on a 5x5 playfield, putting three (or more) of the same colour in a row. This is called a Klax.
Once you get a Klax it will disappear, creating much-needed space on the playfield. Horizontal and diagonal Klaxes are worth more points, as well as multi-Klax combos. Naturally, you can't miss grabbing hold of more than 3-5 blocks, or it's game over time.
The device you're controlling allows you to hold multiple blocks at the same time and throw a block back to the belt for a while.

At least for me, the best part of the game are the different goals in each level, for example 5 diagonals or 25000 points. This makes Klax much less monotonic and repetitive than it could be.

The graphics are bright and flashy, pretty usual coin-op stuff. There are some hilarious sound effects which set a nice mood for the game. I would recommend getting the original arcade version instead of a computer/console clone, although many of them are pretty similar.

Klax can be an incredibly frustrating game. Since it was originally an arcade machine, it's designed to be really hard in order to get as much money as possible from the players. You need to complete the objectives quickly or the constant stream of falling blocks becomes too fast to handle. Luckily, when emulating the coin-op you can just keep inserting virtual cash for retries.

The tips shown before each level can be quite dubious.
My personal favorite: Play Klax every day!