You may well already have this game somewhere on your computer (if you use Windows), but here it is again in a place where you can actually find it!
Select the level from the drop-down box (Easy is a 8x8 grid with 10 mines, Medium is a 16x16 grid with 40 mines, and Hard is a 30x16 grid with 99 mines), and click 'New Game' to start.
Click on the squares to open the area, and if you click on a square which has mine you blow yourself to kingdom come (figuratively speaking!), and the game is over.
If you happen not to blow yourself to smithereens, however, the number which appears in the quare tells you how many mines there are in the squares that surround that one, and you have to figure out from that which is the safest square to click on next.
Sounds easy enough, eh?