They differ from pharmaceutical hormone replacements because their chemical structure resembles the natural human hormones. By contrast the pharmaceutical ones, usually derived from pregnant horses, have a very different chemical structure from the human hormones.
Bioidentical hormones are very safe and can be made into creams (most popular) or pills, according to the individual preference. Since they are medications, a regular prescription from a physician is required.
Hormone replacement is not unique to women; men can suffer from lack of testosterone as well. Natural testosterone is available from compounding pharmacies.