Selective focus, aka, "shallow depth of field", is a compositional, photographic device that sends the viewers eye right to the subject. It's an easy way to create atmosphere and intention within your photos. Here are 3 easy ways to achieve this look.
1) For a DSLR camera, use the lowest aperture lens/settings as possible. Your f-stop should be around 1.4, 2.8 or as low as your lens allows. Set your lens on auto focus and locate your subject, press the shutter halfway down and fire away. The ISO and shutter speed will vary depending on the lighting conditions, but a ballpark for a cloudy day would be 1.4-2.8 f-stop, 125 ISO and 100 shutter speed. Make sure your subject has a little distance between it and the foreground for maximum focus and soft blur contrast.
2) Using a point and shoot, this look can be achieved using your "macro" setting. Just steady your camera/hands and zoom all the way into your subject, hold half way for auto focus/macro settings to kick in, and boom. You've got that similar look -- not quite as dreamy as a DSLR but real close.
3) Tilt-shift. With your phone, you can create this look with a myriad of apps, my favorite though is the "tilt shift" within Instagram. It's not perfect, but for a quick focus/blur look, it can be useful.
Are you a fan of the selective focus? What do you like to use it for? Portraits, food, flowers?