The device is similar to the PDA we know today. Although the PADD is more similar, the Tricorder is a scientific device used to scan an area from anything to special waves or particles to alien virus' and can also be used to collect or transfer data.
On the show's three distinct varients of the Tricorder exist. The standard issue tricorder is a general use device that is given to all Starfleet officers and crewmen even in the original series. The medical tricorder is used specifically by doctors on the show to help diagnose diseases and collect bodily information about a patient. The key differences between this and a standard tricorder due to it using an additional hand held scanner that is moved close to the patients soon-to-be-scanned area. And finally the rarely seen but heavilly documented engineering tricorder. This device is fine tuned for starship engineering purposes.
It is important to note that there are countless other special use tricorders.
In the original series the tricorder appeared to use a CRT like screen and was the size of a fair sized purse complete with strap. Later updates of the device as noted by their "Mark" have made the device into a mid 1990's handheld cellular phone shaped device with a flipout part that turns the device on. The latest "Mark" of the Tricorder is seen on Star Trek: Nemesis the 10th film inspired by the series. It appears to be much more "PDA" like in appearance.
However, a tricorder was in fact created. While many Star Trek fans were very enthusiastic about the device, it turned out to be a legitimate science tool that measued things such as Electromagnetic Levels to colour changes. Although in tradition with Star Trek a stardate clock was built in. It was affectionately named the "Tricorder Mark I" but small numbers of the unit were created with no further developments. The device can occasionally be found in online auction websites but usually fetch a premium. It came with Star Trek documentation references as it was officially licensed and somewhat marketed to appeal.