Countable and Non-count Nouns

Most nouns are countable. That means you can count them:
3 chairs
a dozen eggs
a computer
an optical scanner
You can use numbers with them or the article a or an.
When you make them plural, you generally add an s.
satellites
modems
images
However, some nouns are uncountable, sometimes called non-count nouns:
information
knowledge
water

They cannot be pluralized. They should not have an s or the article a or an under normal circumstances.