Cats are contemplating

Meowing is all-purpose; your cat may be using "meow" as a greeting, a command, an objection or an announcement. Some people have observed their cats walking around the house meowing to themselves.
Chirps and trills are how a mother cat tells their kittens to follow them. Aimed at you, it probably means your cat wants you to follow them, usually to their food bowl. If you have more than one cat, you'll often hear them converse with each other this way.
Purring is usually a sign of contentment. Cats purr whenever they're happy, even while they're eating. Sometimes, however, a cat may purr when they're anxious or sick, using their purr to comfort themselves, like a child sucking their thumb.
Growling, hissing or spitting indicates a cat who is annoyed, frightened, angry or aggressive. Leave this cat alone.
A yowl or howl (they sound like loud, drawn-out meows) tells you your cat is in some kind of distress—stuck in a closet, looking for you or in pain. Find your cat if they're making this noise. However, in unaltered cats, these sounds are part of mating behavior. If your cat is elderly, they may howl because they're disoriented, especially if suffering from a cognitive disorder, such as dementia.
Chattering, chittering or twittering are the noises your cat makes when they're sitting in the window watching birds or squirrels. It usually translates to excitement ... or they may be contemplating snack time.