Estimation is when you make a reasonable guess about a number, usually close to the actual value, while rounding is when you change a number to the nearest whole number, 10s, 100s, 1000s, etc. For example, if you had a number, like 57.6, as an estimate you might say 60, without going into decimals, while when you round the number, you would change it to the nearest whole number, in this case 58.