There are three degrees of comparison - positive degree, comparative degree, and superlative degree. You don't need to know these terms, but you need to know how to use each in a sentence.

The positive degree is the normal adjective form, the one you probably think of when you think of an adjective.
You use this form when describing one thing or a group of things.

For example:

Jessica's backpack is really heavy!
Heavy is an adjective in the positive degree.

The comparative degree is used to compare two people, places, or things.
The word "than" typically follows the comparative form.

For example:

Heavier as a comparative form - it compares Susan's backpack and Pedro's backpack. The word "than" follows the comparative form heavier.

Susan's backpack is heavier than Pedro's backpack.

The superlative degree can be used if you are comparing three or more people, places, or things.
The word "the" typically comes before the superlative form.

For example:

Heaviest is the superlative form - it compares the backpacks of all of the students.

The heaviest backpack belongs to Luke, because he never cleans it out!

You make comparative forms in the following ways:

by putting more/less before the adjective
if the adjective is one syllable long: by adding -er to the end
if the adjective is two syllables long and ends in -y: by changing the -y to -i and adding -er to the end

You make superlative forms in the following ways:

by putting most/least before the adjective
if the adjective is one syllable long: by adding -est to the end
if the adjective is two syllables long and ends in -y: by changing the -y to -i and adding -est to the end

Note: Careful! Only do ONE of these two ways. Phrases like least smartest, more hungrier or most tallest are not grammatically correct!

The handy chart below can help you remember the rules to form comparative and superlative degree of adjective.

Rule	Positive	Comparative Form	Superlative Form
One-syllable adjectives	____ small cool big	____+er smaller cooler bigger	____+est smallest coolest biggest
Two-syllables adjectives ending in -y	____ lazy pretty	____change "y" to "i" + er lazier prettier	____change "y" to "i" + est laziest prettiest
Adjectives with two or more syllables	____ intelligent charming	more/less ____ more intelligent more charming	most/least ____ most intelligent most charming
Irregular adjectives	good bad	better worse	best worst

For example:

There aren't many animals at the Smallville Zoo. The Big Town Zoo has more animals, but the Giant Opolis Zoo has the most!
I don't find math class boring at all - I think history is way more boring. But my friend Lisa thinks math is the most boring class she has.
Leah is pretty intelligent, but Natasha is more intelligent than her, and Kara is the most intelligent of all!

Summary

The positive degree is the standard adjective we use when describing one thing without making a comparison.
Comparative form is used when you are comparing two people, places, or things.
The word than typically follows the comparative form.
Superlative form can be used if you are comparing three or more people, places, or things.
The word the typically comes before the superlative form.
The handy chart below can help you remember the rules to form comparative and superlative degree of adjective.

Rule	Positive	Comparative Form	Superlative Form
One-syllable adjectives	____ small cool big	____+er smaller cooler bigger	____+est smallest coolest biggest
Two-syllables adjectives ending in -y	____ lazy pretty	____change "y" to "i" + er lazier prettier	____change "y" to "i" + est laziest prettiest
Adjectives with two or more syllables	____ intelligent charming	more/less ____ more intelligent more charming	most/least ____ most intelligent most charming
Irregular adjectives	good bad	better worse	best worst