Rounding to decimal places
Calculations may often give an answer with lots of decimal places. Having all of these numbers after a decimal point makes numbers difficult to work with, and are sometimes unnecessary. Rounding a number gives an approximate answer that is "good enough" to work with and makes further calculations easier to do.
Rounding to decimal places is exactly like rounding with whole numbers. The number of decimal places tells you how many digits should be after the decimal point. However, you can't just chop off the rest of the digits  you need to make sure you have rounded properly, using the usual "5 or more, round up" idea. 3.248 rounded to one decimal place would be 3.2. There is now one number after the decimal point. We know that the answer should stay at 3.2 and not round up to 3.3 by looking at the digit in the second decimal place. In this case, it is 4, so we stay at 3.2 3.248 rounded to two decimal places would be 3.25. There are two numbers after the decimal point. We know that the answer should round up to 3.25 and not stay at 3.24 by looking at the digit in the third decimal place. In this case, it is 8, so we round up to 3.25. 
First Steps 
More Like This 
Next StepsRounding to significant figures

External links
 BBC Bitesize: Notes and a couple of questions to try.
 IXL: Online questions with solutions.
 Manga High: Online game/questions on rounding to any number of decimal places.
 Mymaths: Online lesson on rounding to one, two or three decimal places; test yourself with the online homework.
 Transum: A selection of selfmarking exercises on rounding. Use the links to select the level of difficulty you need.