Relative- Can determine the age of the fossil with respect to another rock/fossil. It's performed by comparing depth of fossils. Source is stratum to the position of a reference rock/fossil.Drawbacks: Imprecision and limitation of age date.
Absolute- It can determine the age of a fossil in years. It's performed by determining the relative amounts of a radioactive isotope in a specimen. Drawback: Difficulty of radioassy laboratory methods.
Half life- At the instance the matter is formed, it's 100% radioactive. In about 1.2 billion years, it'll only be half radioactive. In another 1.2 billion years, it'll be one fourth radioactive and so on.