One of the things that make this game an outstanding classic of all time are the basic elements that can create a smash: simple to play, hard to master, and addicting gameplay also helps, all of which Tetris has.
Imagine, if you will, a game of blocks: everything's in regards to blocks here. This is like those alphabet blocks your parents probably had you play with as a kid, although you use these blocks to balance out lines, rather than spell words. Everything about this game is so blockular (oh yay, I invented a new word) that all of the pieces that make up ANYTHING in this game have four blocks each ('count em! I did! Yes, I lead a very exciting life!). And being called a "blockhead" would probably be a complement meaning that you're good at this game, I suppose.
Now, imagine blocks falling from the sky (and no, not in the case of the doomed satellite Skylab or anything, I mean as far as the world of video games go). Blocky shapes will drop down from the top of the screen (nope, no fancy polygon-shaped images here), and you must position them correctly so they all line up horizontally, which will erase a line, and if you erase the required amount of lines per level, you will advance to the next level then, plus you'll get a bonus for completing a low puzzle (i. e. for not having a lot of pieces left over when you clear a screen). So think of this as "building for points", especially since there's designs of Russian buildings all over the place in this game, on the title screen, the marquee and all (but no pesky building inspectors to bring you down, though, although they were definitely needed in some other games: how come sub-standard wiring to spark all over the place in Crazy Climber was allowed on those buildings? And lets not even GET into the pest control problem[s], with the birds and King Kong and all).
Like I said, that's simple enough. You also get a preview of what piece will be raining down from the sky and onto your nerves next (shown in the top left hand corner of the playfield), but alas, the pieces are different shapes and sizes, as you get ones that are long and straight, ones that are square, ones that are shaped like the humanoids in the game Defender (I REALLY hate those, they're usually really difficult to fit *anywhere*), etc. Luckily, though, you can rotate pieces around, which can help a piece fit into a place where it wouldn't if you had just let it drop to the bottom of the screen without repositioning it (so if you're so much of a weenie where you're afraid to ask for extra bread at a restaurant, then you should probably skip this game and just stick with Solitaire).
Then other equally cute things happen: over time, the pieces start dropping faster. Levels start out with blocks that are ALREADY THERE at the bottom of the screen; crap. And waiting for the exact right piece to appear to clear out a line is a formula for disaster, which can end your game prematurely (but this happens to probably *all* of us though, from time to time), plus there are also blocks in the later levels that will pop up at random; eech.
Like I indicted earlier, the shelf life on this game has been extraordinary, especially with all the incarnations that have popped up since it's inception to gaming platforms throughout the years. This is a severe classic, so simple that the not-so-fancy graphics (mmmm, blocks! None of my furniture looks like this, and it's pretty obvious why!) don't really matter, the controls don't have to be spot-on or anything (but they respond well enough), although the music is actually pretty decent for such an old game. You can also choose what levels you would like to start on, like on Tempest, or you can just start at the very slow beginning (like I usually do) with no bonus, but that just suits me fine, thanks.
All these years later since it's release, to this DAY I still occasionally play a game of Tetris myself (here in 2004) in the lobby of the dollar theatre that I frequent, where the high score is over 800,000; good LORD! I don't see how a score that high could be possible, but it was achieved somehow...HEY! That guy who was watching me play it last week -- who was LAUGHING, due to my pathetic score of less than 100,000 -- might have *been* him. Feh...I would've taken him down, too, but unfortunately he was nearly a foot taller than myself, so...
And I'll end this stellar achievement in video game history review with this bit of trivia: this was also made into the world's largest video game -- TWICE, at the time of this writing -- as a game of Tetris was broadcast on the side of a building, making the game several stories high. (The biggest Tetris game in the Western hemisphere -- that was broadcast on a building -- can be seen at http://bastilleweb.techhouse.org, and then THE biggest one in the world is somewhere at http://www.dcsc.tudelft.nl, which the exact address is currently not available at press time.)
Talk about larger than life...as is Tetris.