Pixel measuring tool

I t just looks like the same measurement tool used when using the cmd+shift+4 keyboard shortcut for creating custom sized screenshots.

Yes. I believe is the screenshot tool

Yes, it's correct.

You can select a region of the screen by pressing cmd+shift+4 and then holding the cursor pressed while dragging it around. Without releasing the cursor, it makes for a convenient screen ruler that can be opened quickly up at any time. By releasing it you will take a screenshot or you can dismiss it by pressing the Escape key.

Link

Thanks for verifying guys, just wanted to make sure I wasn't missing something dumb, thanks!

@WESLEYA - One other nifty trick is after you've started drawing with the tool, you can press space to move the selection around. For those cases when you want extra precision. If you press space before you started drawing, you will be able to take snapshots of bits of the screen (e.g. the dock, the chrome window, a folder, etc)