Comparisons

Usually comparisons are made by placing plus or moins before an adjective or adverb. 

Comparisons Superlatives Equality
vite
quickly
plus vite que
more quickly than le plus vite

the most quickly/ the fastest

aussi vite que as quickly as
moins vite que
less quickly than le moins vite
the least quickly / the slowest

Arlette parle moins vite que Jeanne, mais plus vite que Nadine.
Arlette speaks less quickly than Jeanne, but more quickly than Nadine.

Irregular forms

Adverbs Comparisons Superlatives 
bien
well mieux
better le mieux
the best
mal badly plus mal worse le pire the worst 
beaucoup
much plus
more le plus
most, the most
peu
little moins
less le moins
the least

Pierre travaille bien, Henri travaille mieux que lui, mais George est le meilleur des trois.
Pierre works well, Henri works better than him but, George is the best of the three.

Marie étudie beaucoup. Jean étudie le moins des deux.
Marie studies a lot Jean studies the least.

Marie a les cheveux les plus jolis de la classe.
Marie has the prettiest hair in the class.

A few adjectives have special forms:

Adjectives Comparisons Superlatives 
bon
good meilleur
better le meilleur
the best

mauvais

bad plus mauvais/pire
worse le pire the worst 

Ce gâteau est bon mais celui-ci est meilleur.
This cake is good but this one is better.

Vos résultats sont plus mauvais que je pensais.
Your results are worse than I thought.

Le temps est mauvais, mais il était pire hier.
The weather is bad, but it was worse yesterday.