A screen cap is a photo from a movie or TV show, whereas a photo is like just a photo of the celebrity/band/whatever from a photo shoot or public appearance.
I think a screencap is when you just take whatever is on the screen and make it into a picture. I mean like for a screencap you'll literally take your camera and snap a pic of the screen, or you'll hit ALT+PrtScn to make a cap of your computer screen. Plus, screencaps are of the entire screen, normally. No cropping or whatever.
I think photos are more edited and planned, rather than a random pic of a random scene.
