Cats use multiple forms of communication to gather information and also to share information about themselves.
One aspect of communication involves body language. Depending upon the situation she's in, her body language can range from subtle to overt. For example, you may watch two cats walk by each other and not pick up on some subtle communication taking place between the two. It might involve the way one glances at another or the position of the ears, among other subtleties. That subtle exchange may say "hi, friend" or "leave me alone."
Being aware of your cat's body language can enhance your relationship with her. You'll be able to interpret when she's saying "come closer" or when she's saying "please don't bother me." A better understanding of body language can also help you determine whether your cat is frightened, feeling defensive, soliciting playtime or affection, or indicating that she's had enough interaction for the moment.
Let's start with something very obvious such as the ears. If your cat's ears are pricked up and forward-facing, it usually means she's on alert. The ears could be in this position when she greets you at the door, hears an interesting sound, or is engaged in playtime. If her ears are in a "T" position she is probably not happy. If you attempt to engage her, it will most likely have a less than positive effect.
In order to accurately interpret body language, you also need to take into account the immediate environment. For example, if your cat is crouched in what is commonly referred to as the meatloaf position, it could mean two different things based on environmental circumstances. The meatloaf position is basically a crouch where the cat tucks her feet underneath her and wraps her tail tightly around her body. If she's in her home, this position may simply mean she's taking a catnap while still remaining ready to pounce on a toy should the opportunity arise. If the cat is sitting on the examination table at the veterinary clinic however, this position means she's protecting her body and indicating that she'd rather not interact with anyone.
There are books available on cat behavior that explain body postures. You can also learn a lot just from watching your cat. There are so many body postures and the more familiar you become with them, the easier it'll be to read what your cat is feeling.