A new enthusiasm for game playing
By now your toddler is probably pretty good at walking, has a handful of words in her vocabulary, and likes to push and pull toys while walking. She can most likely use a spoon or fork and might be able to walk backwards. She probably likes playing games with you, like pointing to body parts or pictures when asked. She also likes taking objects in and out of containers. She's beginning to get a sense of how things fit -- she'll try matching lids and stacking blocks.
Her individual style is developing
Every child is born with a particular way of interacting with the world. This is her temperament. As she grows, these inborn inclinations will begin to show. While one child might tend to be flexible and adapt easily to change, another prefers a more predictable schedule. Work out how your child is likely to respond and how you can modify her environment to help her succeed. Adults can seek out the circumstances that make them most comfortable, but your toddler depends completely on you. Respecting your child's innate characteristics will help her develop her fullest potential.
Awareness of self
At 15 months, your toddler recognises herself in the mirror -- no longer will she reach out and try to touch the "other" baby. She's beginning to experience herself as an autonomous entity, not merely as an extension of you.
Just saying "no!"
Beginning as early as a few months after their first birthday, most toddlers go through a period of adamant no-saying. It's their way of asserting their new sense of self. You might be able to reduce her "no" usage by keeping yours to a minimum. Instead of saying "No, don't touch that," you could try "I'd like you to play over here."