RoHS stands for Restriction of Hazardous Substances and regulates the use of certain hazardous substances in electronic equipment. The RoHS standard is fully implemented in Europe, with lesser restrictions applying in the US.
In particular, while RoHS mostly originated out of concern about the use of Lead in manufacturing, it addresses and restricts the use of not just Lead, but also Cadmium, Mercury, Hexavelent Chromium (a carcinogen), and Polybrominated biphenyls (flame retardants that may cause a variety of health issues) and Polybrominated diphenyl ethers (another flame retardant that may cause a variety of health issues).
The primary difficulty in meeting RoHS standards is that RoHS mandates that each component of a device fulfills the entire RoHS directive. In other words, if a single small subsystem is not in compliance, the entire device will not be considered in compliance. This can mean expensive redesign of almost an entire existing device.
