This page has a very brief overview of Japanese numbers. I wrote it to avoid having to answer individually the many emailed requests I receive.
Numbers in Japanese can either be written in the usual "European" fashion (0...9), or using kanji. 0...9 is more often used when writing horizontally, kanji when writing vertically, although there are plenty of exceptions to both.
When using kanji to write numbers, a positional system is employed. 25 is "two ten five", 1246 is "thousand two hundred four ten six", etc. Note that there is a character and word for 10,000 (man), but not for a million. To say a million, you have to say "one hundred ten-thousand".
The attached example shows the basic kanji and how they are pronounced. Note that for the numbers 1-10 there are two sets of pronounciations, which have different roles in Japanese (a bit too complicated to explain here.)
Click here to see the set of kanji used for numbers.