Photos for Mac is part of OS X Yostemite. To get the app, make sure you're on OS X Yosemite and have the latest updates installed.
iPhoto was a terrible application. It was slow, buggy, and rarely updated. Thankfully, Photos for Mac is none of these things and is actually a great photo editing app for most people. However, professionals will want to look elsewhere.
The beat features from iPhoto
Photos for Mac distills iPhoto's features down to the ones that matter and improves on simple editing tools. Photos can be as idiot-proof as you'd like or you can dig into more advanced settings to get the results you want.
There's an auto-enhance tool as well as an auto-crop tool that analyzes your photos for horizons and straightens them. This feature makes bulk edits for photos a breeze.
There's also sliders for adjusting lighting, which adjusts many settings hidden from the user. If you want to tweak this automation, you can with the drop down menu. You still have access to exposure, highlights, contrast, and more.
Faces and Places made it to Photos for Mac but they're hidden. You can tag faces by clicking on the info button for each picture. The app's facial recognition software makes tagging people a breeze. Places is now hidden until you zoom out all the way in the photos tab. You'll see locations in the sections for each year. Clicking on a location will bring up a map that stacks photos on each location you visited. However, you're no longer able to manually geo-tag photos, which is a bummer.
Learn the new interface
If you've been using iPhoto for a long time, Photos may look like a dramatic departure. However, I found it intuitive to pick up and to find the adjustments and settings I was looking for. There is a slight learning curve but you don't have to spend too long with it to find out everything.
Sharing is easy too, though Flickr and Facebook sharing has been removed. Now you're treated with Yosemite's iOS-like sharing menu.
If you have a big photo library, you'll want to pay for more iCloud storage. It's not too expensive and you'll get Photos for Mac's killer feature: cross-device sync of all your photos. If you edit a photo on one device, it's instantly available to all your other devices in full resolution.
A great iPhoto replacement
Apple did a great job balancing simplicity with more advanced features. Photos for Mac simplifies editing, which should please most users. Professionals were likely using other apps instead of iPhoto for editing and Photos for Mac isn't going to win them over either. I recommend professionals purchase Adobe's photography subscription, which gets you Lightroom, Photoshop, and a bunch of mobile apps as well.