Fantastic, informative guide by Rene Ritchie, as ever:
From magic wand, to basic color, light, and black & white tweaks, or full, granular control over exposure, saturation, intensity, and more. You can also rotate, flip, crop, and straighten, remove redeye, touch up blemishes, and more. Photos for OS X has everything you need to make your pictures look exactly how you want. What’s more, all the edits are non-destructive, so if you don’t get something perfect the first time, you can change it again whenever you like, or even go right back to the original. Combine that with the large screen, and editing photos on the Mac isn’t just easy, it’s accessible to everyone.
And if you’re also interested in the cloudy side of things, don’t forget to check out his equally impressive guide to iCloud Photo Library. Great stuff.