For example: 1)These green grapes are good.
                       2)Horses can run fast.
                       3)Pear trees can grow tall.
                       4)This flower shop has a large variety of flowers. 
All the underlined words in the above sentences are describing something .

There are two forms of adjectives that are used to compare things. They are called :

1)Comparatives - Comparatives are used to compare two things.
For example: Horses are fast but trains are faster.
Here two things were compared. Fast is an adjective. Faster is its comparative form

2) Superlatives - Superlatives are used to compare more than two things.
)For example: Pear trees are tall, but pine trees are taller and redwood trees are the tallest.
Here more than two things were compared. Tall is an adjective, taller is its comparative form
and tallest is its superlative form.

All adjectives can be written in comparative and superlative forms. However there are
some basic rules to follow.
1) Comparative adjectives usually end in er . Example:  Fast  –  Faster.
2) Superlative adjective usually ends in  est. Example: Tall  - Taller – Tallest.

3) If an adjective is ending with an e, just use r and st. Example: Wide – Wider – Widest.

4) If the adjective ends in a y change the y to I and then add er and est.

Example: Happy – Happier- happiest.

5) If the adjective is a long word use more and most or less and least before the word instead of changing it to er and est.

Example: Beautiful – More beautiful – Most beautiful.

Expensive - Less expensive - Least expensive.

There are two important exceptions when the entire word changes.

1) Good – Better – Best.

2) Bad – Worse – Worst.