To compose a new email in Gmail, you (of course) click on the Compose button, which opens the popup editor:
Sometimes, though, you want to open the editor without it sitting on top of your gmail interface – perhaps so you can refer to another email while composing.
By holding down Ctrl when you click Compose, the editor will open instead in a new browser tab. Alternatively, hold Shift when clicking Compose and the editor will open in a new window altogether:
You can use the same trick to open individual mail conversations in a new tab or window; Hold Ctrl or Shift and click on an individual email in your list to open the email conversation in a new tab or window, respectively.