"Content" or "Contents"?

You should be aware where you're using it.

According to ldoce:

• contents [plural] : 1) the things that are inside a box, bag, room etc 2) the things that are written in a letter, book etc

She kept the contents of the letter a secret.

• content [singular] the amount of a substance that is contained in something, especially food or drink

water with a low salt content

• content [singular, uncountable]: the ideas, facts, or opinions that are contained in a speech, piece of writing, film, programme etc

The content of the media course includes scripting, editing, and camera work.

• content [singular, uncountable]: the information contained in a website, considered separately from the software that makes the website work

The graphics are brilliant. It’s just a shame the content is so poor.

The New Oxford American Dictionary entry for content has:

1. (usu. contents) the things that are held or included in something: he unscrewed the top of the flask and drank the contents | he picked up the correspondence and scanned the contents.

• [usually in singular] the amount of a particular constituent occurring in a substance: milk with a low-fat content.

2. the substance or material dealt with in a speech, literary work, etc., as distinct from its form or style: the outward form and precise content of the messages.

So, it tends to be used in the plural, except in certain particular cases.