Femara is the brand name form of the aromatase inhibitor Letrozole, part of a wave of new drugs which were developed in response to the need to not simply displace estrogen at receptor sites in the body (which is how SERMS work) but to actually prevent the formation of estrogen entirely.
Developed by pharmaceutical giant Novartis, Femara is classed as a orally active non-steroidal aromatase inhibitor. Crucially, this means its range of actions is different in the body compared to a drug such as Aromasin, which while still being an aromatase inhibitor also has steroidal action meaning it can cause androgenic side effects.
Femara has been used off-label for fertility treatment for ovarian stimulation while it has also been used by many bodybuilders who prize Femara's ability to suppress estrogenic side effects related to the use of aromatising anabolic steroids such as testosterone. Finally, men with gynecomastia have experimented using Femara to control the spread and even inhibit some of the visual appearance of gyno.
Femara, being non-steroidal is often preferred by women over a drug like Aromasin. Furthermore, one of the reasons it has taken off as a fertility drug is the greater effectiveness and lower incidence of side effects compared to a common fertility drug such as Clomid.
Femara is licensed in the USA only for post-menopausal women with breast cancer. Outside of this use case though it has been employed in a number of different ways as indicated above.
Femara is generally taken once daily with both dosing and how long it is administered dependent upon the specific use case involved.