A unique sort of vocalization, for example a low meow or chirp sometimes with synchronized purring is a cat greeting.
Purring is usually a sign of contentment. Some felines, however, purr when they are in extreme pain, or in labor, merely to try to calm themselves down. Purring consequently can be a sign of pleasure or pain; more often than not it is the former. To this day, scientists have not yet been able to discover exactly how purring works, but it is suspected that it is caused by minute vibrations in the voice box. Even lions and tigers purr...
Mewing is an appeal for help or attention often made by kittens in peril. There are two basic forms of this call, one more loud and frenzied, the other shriller and penetrating... In older cats, it is more of an unnerved repeated meow.
Frequently simple meows and mews in both senior cats and young kittens are calls for attention. A powerful meow is a command for notice, food, or to be let out.
Cries of protest are whining, complaining or half meow/half growls...
A strong heave or sigh or exhaled grunt shows annoyance or irritation.
A meow that starts out low then goes up and then comes back down like a song is a contented meow.
Cats will often chirrup or chatter upon seeing something of interest such as prey... This is often attributed to mimicking a birdsong to attract prey, even when they see squirrels or mice.