Just so this isn't confusing, Now Trending is something I plan on making a semi-regular series to discuss current design trends.
For the first Now Trending, we're going to look at ghost buttons. A ghost button can actually be seen in my post on Things that don't look like buttons, but they are becoming more and more prevalent. What is a ghost button? It's a button that is made to blend in with its environment while still looking like something you can press - this generally means text with some sort of button-shaped border.
Some ascribe the boost in popularity to the design of iOS7 (as some do with literally any flat or gradient item), but while iOS7 may have made them more popular, it doesn't seem that Apple invented the concept.
Ghost buttons are part of a larger design trend that we'll cover later, focused on typography and minimal lines on top of compelling backgrounds, be they solid or photographic.
The place where ghost buttons probably work the best is in an interface where the designer wants to emphasize or de-emphasize a certain action or behavior, as seen above. Ghost buttons look like buttons, so the user knows they can click, but they are better suited to secondary actions.
Interfaces with two or more ghost buttons and no "actual" buttons have a weaker "software voice," so the user doesn't perceive any suggestion as to what they want to do. Besides that, buttons get very repetitive very fast.
If you want to see an exhaustive list of examples of the ghost button in use, check out this Tumblr.