What is XML?

eXtensible Markup Language

 ... a language for manipulating and communicating information,
for example...
<LandingCall>   <Floor>1</Floor>   <Direction>Up</Direction> </LandingCall>

Information can be complex:
<LiftInformation> <DynamicData> <Group ID="1"> <Car ID="1"> <Status>InService</Status> <Floor>1</Floor> <Direction>Up</Direction> <RegisteredCalls> <LandingCall> <Floor>20</Floor> <Direction>Down</Direction> </LandingCall> </RegisteredCalls> <DoorState>Closed</DoorState> <Load>1</Load> </Car> <Car ID="2"> <Status>InService</Status> <Floor>5</Floor> <Direction>Down</Direction> <RegisteredCalls> <CarCall> <Floor>10</Floor> </CarCall> <LandingCall> <Floor>8</Floor> <Direction>Down</Direction> </LandingCall> </RegisteredCalls> <DoorState>Closed</DoorState> <Load>1</Load> </Car> <Car ID="3"> <Status>InService</Status> <Floor>15</Floor> <Direction>Up</Direction> <DoorState>Closed</DoorState> <Load>1</Load> </Car> <RegisteredCalls> <LandingCall> <Floor>20</Floor> <Direction>Down</Direction> </LandingCall> <LandingCall> <Floor>5</Floor> <Direction>Down</Direction> </LandingCall> <LandingCall> <Floor>6</Floor> <Direction>Down</Direction> </LandingCall> <LandingCall> <Floor>5</Floor> <Direction>Down</Direction> </LandingCall> <LandingCall> <Floor>6</Floor> <Direction>Down</Direction> </LandingCall> </RegisteredCalls> </Group> </DynamicData> </LiftInformation>

• Structure
• Lists
• Optional values

XML is text so is "human readable".
It can also be transformed (for example to graphics) to make it easier to read: