Basically, two systems are followed for writing the numbers.
Simply put, in one system, the numbers are grouped into three digits starting from the left. Each group is referred to as hundreds, thousands, millions, billions etc. (Each group is related by a common multiplier 103).
In the other system, the first three digits from the left are grouped, and then the remaining are grouped in two. Each group is referred to as hundreds, thousands, lakhs etc. (Each group is related by 102).
Eg: 345000 can be written as 345,000 (1st form) or 3,45,000 (2nd form). In the first case, it is read as Three Forty Five Thousand. In the second case, it is read as Three Lakhs Forty Five Thousand.
There are also many other ways to represent a number,for eg. exponential form, Roman numerals etc.