Don't know what is it?? Let me describle.
It's a cube with either 4 squares, 9 squares, 16 squares or 25 squares on each side, marked by different colours and normally you spend a minute messing up all the colours and spend the rest of the year trying to put everything back to the same colour.
Got an idea what is it? NO??? Then better scroll down further. I promise you will say "Orh... that thing har..." after you scroll down.
The most common you see is actually a 3 x 3 cube.
"A normal (3×3×3) Rubik's Cube can have (8! × 38−1) × (12! × 212−1)/2 = 43,252,003,274,489,856,000 different positions (permutations), or about 4.3 × 1019, forty-three quintillion (short scale) or forty-three trillion (long scale).
Despite the vast number of positions, all Cubes can be solved in twenty-six or fewer moves .
To put this into perspective, if every permutation of a Rubik's Cube was lined up end to end, it would stretch out approximately 261 light years. If they were laid side by side, it would cover the Earth approximately 256 times.
In fact, there are (8! × 38) × (12! × 212) = 519,024,039,293,878,272,000 (about 519 quintillion on the short scale) possible arrangements of the pieces that make up the Cube, but only one in twelve of these are actually reachable. This is because there is no sequence of moves that will swap a single pair or rotate a single corner or edge cube. Thus there are twelve possible sets of reachable configurations, sometimes called "universes" or "orbits", into which the Cube can be placed by dismantling and reassembling it."
Taken from http://en.wikipedia.org/wiki/Rubik
If it sounds cheem to you using the normal 3 x 3 cube, now, watch this...