Depending on the type of adjective and the context, it may occur either before or after the noun it modifies. Most frequently, adjectives follow the noun unless they are limiting adjectives or used metaphorically or modify a noun which is one-unique.

Limiting adjectives (adjetivos determinativos) are normally are placed in front of the noun. These include adjectives which indicate

• quantity [e.g., mucho(s), poco(s), cuanto(s), todo(s), dos, etc.],
• articles (el, la, un, una, etc.),
• unstressed possessives (mi, tu, su, etc.),
• demonstratives (este, ese, aquel, etc.),
• moral qualifiers (buen, mal, etc. if not preceded by adverbial modifiers such as muy) and particularly the comparative/ superlative forms such as mejor, peor):

Pocos mexicanos han leído mis libros.
Few Mexicans have read my books.

Estos regalos son para los mejores niños.
These gifts are for the best children.

Todas las chicas son alumnas muy buenas.
All the girls are very good students.

Descriptive adjectives (adjetivos calificativos). Adjectives which describe nouns —giving characteristics, for example color and size— normally are placed after the noun.

¿Puedes prestarme tu falda roja?
Can you loan me your red skirt?

El chico alto es mi sobrino.
The tall boy is my nephew.

Los estudiantes brillantes salieron bien en los exámenes.
The brilliant students did well on the tests. (The others didn't do so well.)

It's important to realize that the final (or last or post-) position in Spanish is generally the position reserved for the element which is the most stressed or most important or most distinctive; when an adjective occurs in the post-position it has the effect of distinguishing the noun involved from other such nouns. In the the last sentence given above the phrase estudiantes brillantes occurs, with brillantes in the post-position. This implies a contrast with other estudiantes which are not brillantes (and the sentence then implies that the not-so-brilliant students did not do so well on the test). In contrast, if we wish to indicate that ALL the students were brilliant (and hence all did well on the test), or if we want to characterize the entire group as being brilliant, we would place the adjective before the noun:

Los brillantes estudiantes salieron bien en los exámenes.
The brilliant students did well on the tests.
[That is, (all) the students —all of whom were brilliant— did well.]

Note that when a given noun is unique or one-of-a-kind, the post-position in normally required for adjectives:

Mi esposa hermosa - My beautiful wife: I have more than one wife; I'm referring to the one who is beautiful.
Mi hermosa esposa - My beautiful wife: I only have one wife, and she is beautiful.

Several descriptive adjectives change their meaning depending on whether they are used before or after the noun:

                before                            after 

antiguo         former                            ex- ancient, old  
gran, grande    great                             big, large  
medio           half (a)                          average  
nuevo           new, different                    (brand) new  
pobre           poor, unfortunate                  poor, penniless  
puro            pure (just, merely, all)           pure (clean)
viejo           old, long-standing                 elderly 
único           only                               unique  

Examples:

mi antiguo maestro - my former teacher
mi maestro antiguo - my ancient teacher

un gran presidente - a great president
un president grande - a big president (large man)

media botella - half a bottle
una botella media - an average bottle

mi nueva casa - my new house
mi casa nueva - my brand-new house

una pobre mujer - an poor (unfortunate) woman
una mujer pobre - a poor (penniless) woman

pura leche - mere (or just) milk [for example, not alcohol]
leche pura - pure ( or uncontaminated) milk

un viejo amigo - an old (long-standing) friend
un amigo viejo - an old (elderly) friend

el único ejemplo - the only example
un ejemplo único - a unique example