Decimals are fractional numbers indicated by digits after a period called a decimal point. When lining up decimals, fill empty spaces with zeroes and compare values from left to right. For rounding, look to the right of the rounding place and round up if the number is five or greater, otherwise keep the same value and change trailing digits to zero. When adding or subtracting decimals, line up the points and bring the decimal down in the answer. For multiplication, count decimal places and put the point at the end, moving it the same number of places. For division, move decimals to the end of each number and divide like whole numbers, adding zeroes if there is a remainder.