Adjectives

Adjectives describe nouns.  They help us identify or quantify an noun.  As we can see in this section from Jack Kerouac's On the Road, adjectives generally appear before the noun that they describe, but they can also appear after the noun, particularly with forms of the verb to be:

Her hair was long and lustrous black; and her eyes were great big blue things with timidities inside.  I wished I was on her bus. A pain stabbed my heart, as it did every time I saw a girl I loved who was going the opposite direction in this too-big world.

Possessive adjectives (my, your, his, her, its, our, their) describe to whom an item belongs.   If we look at that passage again, we can see that "her" describes "hair," the second "her" describes "eyes," the third "her" describes "bus," and "my" describes "heart":

Her hair was long and lustrous black; and her eyes were great big blue things with timidities inside.  I wished I was on her bus. A pain stabbed my heart, as it did every time I saw a girl I loved who was going the opposite direction in this too-big world.

Demonstrative adjectives (this, that, these, those) specify which object and how far away that object is.  Again, if we look back to the Kerouac passage, we can see that "this" tells us which "world":

Her hair was long and lustrous black; and her eyes were great big blue things with timidities inside.  I wished I was on her bus. A pain stabbed my heart, as it did every time I saw a girl I loved who was going the opposite direction in this too-big world.

As always, Schoolhouse Rock has a good video for adjectives:

Resources

 The University of Ottawa, Towson University, and Capitol Community College have good pages on adjectives in general.  University of Victoria has a good page on the order of adjectives.

Purdue OWL has a good exercise on distinguishing between adjectives and adverbs.  Englishmaven has a test on identifying adjectives in a sentence.