For years now, Sony's been iterating its mirrorless line of cameras one tiny tweak at a time. That continues today with the A5100, a minor variation on the company's existing line.
The A5100 replaces the NEX-5T, Sony's current midrange mirrorless shooter. It's the last of the Sony NEX cameras, which will all simply be part of the Alpha line from now on. It's also a very minor upgrade over the existing A5000. It inherits that camera's body design, but adds the more advanced image sensor from the A6000. The A5000 is a teeny tiny little guy—the smallest APS-C interchangeable-lens camera out there—but before this upgrade it didn't have the snappy hybrid autofocus system that's being used by many cameras these days. That changes now because the A5100 gets the 24.3-megapixel APS-C sensor with combined phase and contrast autofocus detection. Not bad for $700 when bundled with a 16-50mm lens.
If it feels like all of these Sony mirrorless cameras are starting to blend together a bit, you're right. Sony's A6000 was a great camera for the money, and you can expect similar performance in a smaller body now. Sony's mirrorless cameras have always been solid. There's nothing new about that.