From The American Heritage Book of English Usage:
We often use adjectives -- words that modify nouns -- to make comparisons. We say That building is bigger than this one, She is the most intelligent student in the class, and so on. Some adjectives add -er and -est to form the comparative and superlative degrees. Others cannot do this, but must be preceded by more and most. How can you know which is which? Fortunately, there are some simple rules you can follow. Adjectives that have one syllable usually take -er and -est. Adjectives that have two syllables and end in y (early), ow (narrow), and le (gentle), can also take -er and -est. Almost all other adjectives with two or more syllables require the use of more and most.
English also has a few adjectives whose comparative and superlative forms are irregular: good, better, best; bad, worse, worst; little, less, least.
You can also compare adjectives in a decreasing way by using less and least: Jack is less skillful at carpentry than Bill is. Roberta is the least likely employee to have complained about working conditions.
There are also some adjectives, like acoustic, biological, and reverse, that cannot be compared and others, like unique, parallel, and perfect, whose comparison is controversial.