Is 'is' or 'are' for plurals?

It depends on the syntax of the sentence, but is usually the case that you will use "are" for plurals, and it agrees with the tense of the sentence.

"The chickens are destroying the sofa"

But then it could be two seperate clauses, in which case you will use "is" for the first clause of it is in the singular tense, and the second clase, being plural, you would use are.

"The case is, that the chickens are destroying the sofa"

Problems arise with collective nouns. You can say 'The committee is' or 'The committee are' nowadays. Previously, 'is' was the only option. The thing is to be consistent. If you say 'The Committee is' in the first sentence, you have to keep to that throughout. (Similar problems come with possessives - does the orchestra rise to 'its' feet or 'their' feet? If you are using 'is' for the orchestra, it must be 'its feet' to keep the singular. Note too that orchestras and teams and so on are neuter and use 'its' not 'his' or her'.
Also, 'All sorts of machine' will be followed by 'are' not 'is' as the verb refers to the subject 'sorts' not the 'machine'.
[Ares are a measure of area especially land in the metric system. Strictly the question should read "Is it 'is' or 'are' for plurals?". We know what you meant, anyway!]

Although I see both 'is' and 'are' used with collective nouns, something like "The committee are" will never sound correct to my ears. Even though a committee is made up of more than one person, as a whole it is a singular unit and should take the singular verb. You would not say, "The committee are made up of more than one person."

Now, it seems to get even stickier when you want to say something like, "The committee is casting their votes" because "their votes" is incorrect. Even though there is more than one individual vote, "the committee" is the subject of the sentence and is still singular: "The committee is casting its vote." The only way of making it plural is to break the singular unit up: "The committee members are casting their votes."

And Flynn, you need to shoo those chickens out of the living room! The flock is destroying the sofa!

I'm home now and looked this up in my English handbook, and here is what it says.

A collective noun is singular in form but names a group of persons or things.

Use a plural verb with a collective noun to indicate that the individual parts or members of the group are acting separately. Use a singular verb to indicate that the group is acting as a unit.

The class have completed their projects. [The class is thought of as individuals.]
The class has elected its officers. [The class is thought of as a unit.]

So, I am partially wrong up there. (Ugh.)

The committee are casting their votes, whether it sounds good to me or not. I would probably avoid the situation anyway and go with committee members.

I'm sorry if I caused trouble yesterday. If this is about the chicken question...

Is there any milk left? There is milk left.
Are there any cookies left? There are cookies left.

Things get mixed up when the subject of the sentence is separated from its verb. *There* (the word) is not the subject at all, but an expletive (in grammar that basically means a placeholder) that allows you to move the real subject--whether it be milk, cookies, or chickens--to the end of the sentence for a question or for emphasis. (It's called inversion, meaning backwards or upside down, because the subject usually comes first.) The verb agrees with the subject, because that's what the verb is talking about.

Forget about the sofa, look what the flock did to the carpet...

This question beat the chicken question. It originally read something like "Is is or are ares for plural?" I'm not sure 'ares' is really even a word. I mean, I realize he meant more than one are, but like the plural committee, I'd avoid it!

For "The committee are casting--whatever" (how on earth do you pick a pronoun for that?) I would say "The committee members are casting their votes" or "The members of the committee are casting their votes." It sounds natural. The members are the subject of the sentence anyway, and the other phrasing will just sound hopelessly awkward no matter how correct it's supposed to be.

I've just looked it up: words used as words are supposed to be italicized to indicate that use (not available here) and made plural by adding 's (one of the very few times an apostrophe should ever be used to make a plural.) Since I had to look it up, I wouldn't blame anyone for being a little creative or greatly uncaring about it.