1. Descriptive grammar: the systematic study and description of a language. Descriptive grammar refers to the structure of a language as it is actually used by speakers and writers.

2. Prescriptive grammar: a set of rules and examples dealing with the syntax and word structures of a language, usually intended as an aid to the learning of that language. Prescriptive grammar refers to the structure of a language as certain people think it should be used.

Both kinds of grammar are concerned with rules--but in different ways. Specialists in descriptive grammar (called linguists) study the rules or patterns that underlie our use of words, phrases, clauses, and sentences. On the other hand, prescriptive grammarians (such as most editors and teachers) lay out rules about what they believe to be the “correct” or “incorrect” use of language.

Descriptive grammarians ask the question, “What is English (or another language) like--what are its forms and how do they function in various situations?” By contrast, prescriptive grammarians ask “What should English be like--what forms should people use and what functions should they serve?” (...) Modern grammarians aim to describe rather than prescribe linguistic forms and their uses. Dictionary makers also strive for descriptive accuracy in reporting which words are in use and which senses they carry.

"Prescriptive grammar may tell its users that some expressions are incorrect, contrary to actual contemporary usage by the vast majority of users and writers in the vast majority of cases"
(Synchronic English Linguistics, Paul Georg Meyer)

Examples:
"It is I" or "It's me"?
"I wish I were" or "I wish I was"?
"drive slow" or "drive slowly"?
"the man whom I saw"? or "the man who I saw"?
"the man to whom I spoke" or "the man I spoke to"?
"to go boldly" or "to boldly go"? (split infinitive)?
"I wouldn't think" or "I would think not"?
"between you and me" or "between you and I"?

Em homenagem à Denise, aqui vai trecho do meu English Grammar In Use:
[UNIT 37 - "If" and "wish" sentences (present)]
c) In "if" sentences and after "wish" you can use "were" instead of "was":
- If I were you, I wouldn't buy that coat. (or 'If I was')
- I'd go out if it weren't raining. (or 'if it wasn't raining')
- I wish my room were larger. (or 'I wish my room was larger.')
É claro que só se pode usar "was" ao invés de "were" quando o sujeito está no singular; caso esteja no plural, precisa-se usar "were".
Vale comentar que minhas professoras da Cultura Inglesa não eram fãs do uso de "was" com "if" e "wish". Elas torciam o nariz quando ouviam estas combinações. O uso de "were" é mais formal/tradicional, mais agradável aos ouvidos (se não dos Ingleses, pelo menos aos meus, mas isso pode ser influência das minhas "teachers").
Tentando ajudar... :)

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Note added at 1 day23 hrs (2008-11-14 22:57:09 GMT)
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[Sorry, LittleBalu (and everyone else).]
OK. English version...
I have posted above a small extract from my English Grammar In Use (it is in English, so there's no need to write everything again). Here's what follows:
Of course we can only write "was" instead of "were" when the subject is singular; if it's plural (we, they...), we must write "were".
It's worth the comment on my teachers from Cultura Inglesa. They were not fond of using "was" in "if" and "wish" sentences. They'd literally twist their noses when they heard such combinations. Using "were" is more traditional/formal, more pleasant to the ears (if not to English ears, at least to mine, but this may be the influence of my teachers).
Trying to help... :)