When you pet a cat and it lifts its behind, it is inviting you to sniff its anal glands!

Cats have over 30 different sounds to relay their feelings. Cats are sociable and mix with other cats constantly. They don’t live in packs like dogs do. They use very subtle and complex body language and speak with each other. 

There are at least 25 different visual signals used in 16 different combinations used by cats to intercommunicate. The main goal when they use body language is to avoid or end physical confrontation. The body language is broken down into parts- the head, the body, and the tail. 

They must be seen as a whole, though, to understand what the cat is trying to communicate. The eyes, mouth, and ears must also be paid close attention to. Most cat owners adapt and learn some of what their cat is trying to communicate through body language. 

No one understands it all, though. Interestingly, when a cat lifts its behind after you pet it, it is inviting you to sniff its anal glands. It is a way of being friendly and inviting you to be close with them and get to know them. 

(Source)