Although most radicals are highly unstable and quickly react with
other molecules in a chain reaction, some radicals are slower to
react and will on occasions not react at all. There are two types
of radicals known as indeffinatly stable radicals and persisitant
radicals. A short exlantion and some examples of these two forms of
radicals are given below.

Indeffinatly stable radicals are extremely stable due the
exstensive delocalisation of the unpaired electron into surrounding
aromatic rings. This exstensive delocaisation leads to the radicals
having a stability similar to that of other closed shell molecules.
Below are some examples:

Persistant radicals also have a long life time, in this case
however it is due to the steric relationships and hindrance of the
surrounding groups.Below is an example of a persistant radical: