This is still very much in progress, despite the lists I'm making on it. Sorry!
Many trans women, and other trans people, choose to go on a prescription of artificial estrogen, as well as other hormones that will decrease their body's testosterone production. Human bodies, including those of cis people, naturally contain both some estrogen and some testosterone — these are the sex hormones traditionally classified as "female" and "male," respectively. Most women have a larger amount of estrogen in their systems, and it's the other way around for men.
By introducing more estrogen to her body and decreasing her testosterone, a hormone-taking trans woman will shift her chemical balance towards the levels that are more common in women, and away from the levels that are more common in men.
The effects of estrogen prescribed for trans people can include: