Voxer is a smart-phone app. To say it turns your phone into a walkie-talkie is accurate, but it's not precise. Walkie-talkies are synchronous communication devices. That is, someone has to be available to listen to you on the other end in order for communication to take place.
The synchronicity problem is why, when we invented voice communications, that we had answering services, then answering machines, and finally, voicemail.
Texting has become so popular because of the low technical barriers to use. Voxer is like texting voicemail.
On one end, you hold down a button and speak. That message is sent to the recipient(s). The recipient holds down a button to answer, whether they are available when you sent the message or not.
If the recipient is available, that is, if you are using the device synchronously, new messages are played as they arrive. If the recipient is not available, the phone's notification system is alerted and the message is saved on the device for later retrieval, even off-line.
This is a minor game-changer for me. It makes me approach the phone in a new way. I like it.
Like Instagram, I expect it will be purchased by Microsoft or Facebook at some point, once everyone catches on, but before then, this is really cool.