A toy that consumed about four years of my childhood, much to the chagrin of my parental units' feet.
Legos, at their basic level, are a bunch of colored, interlocking blocks of various shapes and sizes. Most common (for me, at least) were red, blue, green, white and yellow. (This is ignoring things like pirate ships and castles.)
They ranged in thickness as well. There was the standard thickness, I'd estimate about a centimeter thick (I don't have one here to measure, as I gave all of my legos to my cousin long ago). There's also a thinner lego, which if I remember right took three of the thin ones to equal the size of a thick one.
As for the XY-directions, I had some that were 1x1, 1x2, 2x2, 2x3 & 2x4; both thin and thick for each dimension set. Now, there were many more legos out there, just from the time period where I owned some, but my parents were rather poor so almost all of my legos were passed to me by another cousin; but the lego types I just rattled off were the basics, and the rest were (usually) built off of them.