Shift is a bit hard to take in hands on the first go, but once you pick it up you just can't stop playing...
The rules are really basic : click on the left, right, upper, or lower part of the screen (or press the direction pad) to have all the blocks who can move in that direction... If you form groups of 3 or more blocks, they'll disappear, etc...
To get huge scores, trying doing combos, not leaving a single round before exploding another group, etc... Easier said than done !