I was recently asked by a colleague for some advice on locators and I shared this cheat sheet with him. It contains just about everything you need to know for formulating locators for use with Selenium (and also Marionette), and includes syntax for both xpath and css selectors.
I hope you find it as useful as I do.
Selenium Locators Cheat Sheet
Please note that I am not the author of the cheat sheet, just someone who finds it very useful.