These pen and ink line drawings use a technique called "one line" or "continuous line", and it has become a popular trend recently. Most of these have been created for logos, websites, book covers and their internal pages. Others for... well just because.
ONE LINE DRAWING
The technique of drawing without taking your pen off the page
Continuous line drawing is one of the most basic exercises that anyone can do. No matter how many times you draw the same subject, it will be different every time.
The loose technique helps you get over perfectionism because it's not meant to be detailed or realistic - your drawing might be simple, complex, messy or out of proportion, and that's ok!
The technique of drawing continuously without taking your pen off the paper, is usually the first exercise students are given in drawing class. That's because it trains you to observe closely at what you are seeing.