- Hungry!
- Bored! (This may also mean "lonely", as in "everybody has gone into the other room and left me here by myself.")
- Going somewhere! (This is a funny one. When they get up, jump off the chair, walk over to the food dish, etc. they really do make a noise. Announcing what they're doing. I suppose it may also mean "Coming through!" -- ie, they're not trying to sneak up on anyone.)
- Good morning. (As soon as they hear me stir in the morning, they jump right up on the bed and greet me.)
- Pet me!
- Don't stop! (This is what they say when you're petting them and then take your hand away.)
- Love you. (This is not a demanding noise like "pet me." It's more of a happy cuddly sound.)
- Put me down. (or, "Stop." Normally they don't mind being picked up. But if they suspect that I am carrying them towards the cat carrier, or intend to shut them in the bathroom temporarily, or do anything to them that they would not enjoy, they will complain.)
- Angry (for when they're fighting--snarls and yowls.)
- Intruder alert! (I wrote about this here. They never make this noise when they're fighting with each other.)
- Playing. (If they run up to each other and fake an attack, for example, they'll make this noise.)
- Bird! (I have heard other cats make this noise. It's kind of a lip-smacking. I think they only do it for birds--not squirrels, for example.)
They each have individual tones of voice, different intonations. Also, they each vocalize about different things. For example, only one of them really ever says "Put me down!" The other one will just struggle silently--but, he is much more vocal when he's hungry. I also think he uses the "going somewhere" noise more than his brother.
Then there is body language. That's a whole other set of "words."