Should I be using Events or Albums? Say I go to the Zoo with my family, take some pictures and then import into Photos. Is that an Event or an Album? I always used Events with iPhoto.
You should have all of your old iPhoto Events, but they are Albums now.
This answer describes a way of manually replicating some of the Events functionality:
(…) you can create an Album from the last imported by selecting all in the Last Imported album and clicking the + icon on the toolbar.
Unless you don't take many photos, replacing the concept of Events with Albums will quickly become unmanageable in Photos. I'd suggest turning on the sidebar and taking a look at the Photos view to see what Apple has replaced it with - an automatically generated stream called Moments. It basically does what auto-split events during import did in iPhoto, except instead of creating a discrete Event (that was basically an album with a different name), it does it on the fly as you're scrolling.
It has its upsides and downsides, but one nice thing is that if you later add additional photos from the same timeframe, you don't have to manually merge Events or anything like that.