Adjectives describe things, so allow more accurate (and interesting!) communication.
The spelling of adjectives frequently changes depending on the gender of the thing being described. Normally an extra -e is added for feminine words.
Irregular adjectives are a pain - they have entirely different spellings for masculine and feminine words.
Adjectives ending -eux change to -euse in feminine form.
Adjectives used with plurals usually just gain an -s at the end. However, those that end in -al change to -aux.
Adjectives can be positioned before or after the noun, depending on what they are!