A decimal is a number after the decimal point that represents part numbers.
eg 2.3 = 2 whole and 3 tenths
eg 45.34 = 45 whole and 3 tenths + 4 hundredths
Decimal places represents the number of digits after the decimal point that a number has or has been rounded to.
eg 2.3467 has 4 decimal places (4dp)
eg 2.9863524 has 7 decimal places (7dp)
eg the number 4.718183 rounded to 2 decimal places (2dp) is 4.72