Eyewitness News is a format that has evolved over the years. It originated in Philadelphia (KYW) and early on in New York, (WABC), emphasizing more detailed storytelling from the perspective of the reporter and their constituents out on the field, providing an Eyewitness accounts on the story rather than it being reported from the desk. It was more compelling and made the story itself more interesting than the traditional news often boring reading that was done prior to that format.
There is also Action News, also originating in Philadelphia (WPVI) which emphasized more faster paced news, with reporters still out on the field, but delivered at a faster pace, and timed, to allow more stories to be told and to cover a wider area. At the time of their debut there were distinct differences especially in the earlier days.
But today, unless you really attention and are laser focused, there are some elements derived from both formats, its a blend. It's really kinda the same thing and Eyewitness News is just a title now rather than a format. Some stories are faster paced, some stories are slower depending on the context. Today, when I watch WPVI and WABC, who pioneered these formats, there is hardly a difference in their delivery and storytelling style maybe a few elements but they have loosened up a little bit. WPVI still kinda holds onto their Action News format with less reporting and more delivery, whereas WABC, embodies the storytelling from the field approach with the anchor introducing the story and the reporters telling the rest of the story.