Hair and hairs are closely related, but they are not used the same way.
Use hair as an uncountable noun when you mean the substance or the mass of it in general. This is the usual choice when talking about someone’s hairstyle, color, texture, or condition.
- Her hair is very curly.
- My hair needs a trim.
- He washed his hair this morning.
Use hairs as a countable noun when you mean individual strands. This often appears when the number matters, or when you notice a few separate strands somewhere.
- I found three hairs on the sink.
- The brush had several long hairs stuck in it.
A simple way to remember it is this: if you can count them one by one, use hairs. If you mean it as a whole, use hair.
Compare these: Her hair looks great today means her hairstyle or hair in general. There are two hairs in my soup means two separate strands.
This difference is small, but it makes your meaning much clearer.