a.) Spring cloud sleuth generates and attaches a trace id into the log statements to be used by tools like Zipkin or ELK.

b.) Spring cloud sleuth generates and attaches a span id into the log statements to be used by tools like Zipkin or ELK.

c.) Trace id is associated with a single request and used to trace a message across service calls

d.) Span id represents multiple steps in a single request, so a trace id can be associated with multiple span ids.

d.) Zipkin is a distributed tracing system that helps getting timing data of each request that is passed between services 

https://spring.io/blog/2016/02/15/distributed-tracing-with-spring-cloud-sleuth-and-spring-cloud-zipkin