Imagine it this way: If you have any siblings, using relative age dating would be like saying, 'I am older than my brother but younger than my sister,' but using absolute age dating would be like saying, 'I am 23 years old.' Since scientists work with many different types of Earth materials (rock, fossils, etc.), there are many types of absolute age dating.
Some types are useful in certain situations and for certain materials, while others are perfect for other jobs.
The radioactive dating relies on spontaneous decomposition into other element.
The spontaneous decomposition is called radioactive decay.
No form of radioactive dating produces exact results.