To use Share Keyboard, you launch the app, and then go to the indicated URL from your computer. You then type your text on the Share Keyboard website, and send it back to your device.
The other full-keyboard option is rooting your Android device, and then connecting a Bluetooth keyboard. Share Keyboard fills a valuable niche by providing a full keyboard to those who don't want to take the risk of rooting their device.
Share Keyboard is by no means a perfect solution. In order to use it, you have to have a computer with you anyway. You also aren't typing on the Android in real time, so it's more like a quicker means of emailing text to yourself.
A conspiracy theorist would say that Share Keyboard could be collecting the text you send to yourself, like a keylogger. In that were in any way true, maybe caving in and rooting your Android device is the safer solution.
source: AndroidZoom via Android Central