Photos for mac is a replacement for iPhoto which enables you to see and access all your photos from all your devices whether you are on a Mac , iPad or an iPhone.
Photos was part of an update to Yosemite and is less of a resource hog than iPhoto was and will require you to upload your photos to iCloud Drive.
Here are some links to assist you with learning about Photos and maximising its use.
Lifehacker : How To Setup And Use Apple New Photos App In OS X Yosemite
How to delete, recover, and eradicate pictures and videos in Photos for OS X