Kekkei genkai (a technique limited to inheritance by blood or Bloodline Limit) are abilities passed down genetically in specific clans. It's possible for a ninja to have more than one of these abilities. Kekkei genkai abilities that work via the user's eye are called dojutsu. Other kekkei genkai include mixing one type of elemental chakra with another, creating a new one unique to the users, which is usually impossible for normal ninja.
Kekkei genkai and their related techniques cannot be taught to or copied by others. However, kekkei genkai can be given to others. Dōjutsu, or other kekkei genkai isolated to a single organ, can be transplanted into the body of another, as was done with transferring Obito's Sharingan to Kakashi. Use of these organs requires much more chakra than someone born with the kekkei genkai. Chakra elements can also be passed down to people outside of the clan, such as Yamato going through Orochimaru's DNA re-modification to acquire the Wood Release technique. However, Yamato's skill in using it is not nearly as powerful as the original practitioner, Hashirama Senju.
Although kekkei genkai are usually genetically shared within a specific clan, sometimes it is unique to one person alone, and that even family relatives do not share it, such as the Wood Release and Shikotsumyaku. Yet other kekkei genkai seem to appear in individuals with no known relation.
There is an advanced variant of kekkei genkai known as kekkei tota.