Relati*ve adjectives describe the relations between animate/inanimate beings.
The English language would tend to use several nouns to describe relations like this – but Serbian is handling this job with the help of adjectives.

For example:
морски ваздух / morski vazduh (sea air)
градски аутобус / gradski autobus (city bus)
 
Ја научим граматику српског језика.
Učim gramatiku srpskog jezika.
(I'm learning Serbian grammar (lit.: the grammar of the Serbian language).)
 
Који је био ваш студентски живот?
Kakav je bio tvoj studentski život?
(What was your student life like?)

The related adjectives occur only in the certain form, are declined, but they cannot be put in forms of comparison.
Relative adjectives can be derived from nouns by adding an ending (e.g.: -ски/-ski; -њи/-nji).

They are formed from nouns.
There are male, female and substantive forms.
These have to be declined.

For example:
Анин брат / Anin brat (Ana's brother)
Маријанова сестра / Marijanova sestra (Marijan's sister)
бакина хаљина / bakina haljina (grandmother's dress)