In prayer there are four type of rakats, they are :
Fard : These are Obligatory and included in all obligatory prayers. Missing it, means that prayer has not been offered.
Sunna Muakadda: "Muakkadah" is that which the Prophet (PBUH) regularly did [and encouraged].
Sunna Ghair Muakadda: That sunna which Prophet (PBUH) did not offer regularly. If you offer it, you will be rewarded, but if not, there is no sin.
Nafl : These are additional rakats which are non obligatory, and if you offer it you will be rewarded, but if not, there is no sin. On Day Of Judgment, if you will be having deficiency in Fard Rakat, Allah may compensate from these.
Witr : These are Wajib, and prophet (PBUH) was always offering it.