What’s the site icon? It’s that little image in your browser tab for this site.
How does it differ from the site logo? The site logo is the image at the top right of every post and page of this site.
The above answers are short, simple, and specific. If you want more detail, I recommend RockPaperSimple’s account of the difference between icon and logo.
For this site, I’m challenged by the site logo decision, and confident in my site icon decision.