The difference between them is that the number of digits is different, and the number on their second digit is also different. 990 is a 3-digit number and 9000 is a 4-digit number. Their first place (9 in the hundreds and thousands places) is 0 in the ones place. Their second digits are 9 and 0 respectively. 990 is the number of tens, 9000 is the number of thousands.