A transaction is a logical unit of work in a software application. The
Transaction errors occur when a request throws an exception in the code path that was taken to fill that request. The number of transaction errors does not equal the number of transactions, because you can specify whether you want to collect, ignore, or mark errors as expected.
Span events are created when New Relic agents, or other tracing instrumentation tools, instrument operations that are part of a distributed trace. See the definition of span.