Three primary variants of the tricorder appear in Star Trek, issued by the fictional organization Starfleet. The standard tricorder is a general-purpose device used primarily to scout unfamiliar areas, make detailed examination of living things, and record and review technical data. The medical tricorder is used by doctors to help diagnose diseases and collect bodily information about a patient; the key difference between this and a standard tricorder is a detachable hand-held high-resolution scanner stored in a compartment of the tricorder when not in use. The engineering tricorder is fine-tuned for starship engineering purposes. There are also many other lesser-used varieties of special-use tricorders. The word "tricorder" is an abbreviation of the device's full name, the "TRI-function reCORDER", referring to the device's primary functions: sensing, computing, and recording.
The tricorder of the 23rd century, as seen in Star Trek: The Original Series, is a black, rectangular device with a top-mounted rotating hood, two opening compartments, and a shoulder strap. The top pivots open, exposing a small screen and control buttons. The ship's doctor uses a variant of this model with a detachable "sensor probe" stored in the bottom compartment when not in use. The probe was actually fashioned from a salt shaker to save costs.  The 24th-century version introduced in Star Trek: The Next Generation is a small, gray, hand-held model with a flip-out panel to allow for a larger screen. This design was later refined with a slightly more angular appearance that was seen in most Next Generation -era movies as well as later seasons of Star Trek: Deep Space Nine and Voyager.
In the post- Next Generation era (Star Trek Nemesis and Star Trek: Elite Force II ), a newer tricorder was introduced. It is flatter, with a small flap that opens on top and a large touchscreen interface.
The tricorder prop for the original Star Trek series was designed and built by Wah Ming Chang, who created several futuristic props under contract. Some of his designs are considered to have been influential on later, real-world consumer electronics devices. For instance, his communicator inspired cell phone inventor Martin Cooper's desire to create his own form of a mobile communication device. Many other companies followed this example and life-sized replicas remain popular collectibles today. The tricorder in The Next Generation was initially inspired by the HP-41C scientific calculator.
Software exists to make hand-held devices simulate a tricorder. Examples include Jeff Jetton's Tricorder for the PalmPilot; the Web application for the Pocket PC, iPhone, and iPod Touch; and an Android version.
Vital Technologies Corporation sold a portable device dubbed the "Official Star-Trek Tricorder Mark 1" (formally, the TR-107 Tricorder Mark 1) in 1996. Its features were an "Electromagnetic Field (EMF) Meter", "Two-Mode Weather Station" (thermometer and barometer), " Colorimeter " (no wavelength given), " Light meter ", and "Stardate Clock and Timer" (a clock and timer). Spokespersons claimed the device was a "serious scientific instrument".  Vital Technologies marketed the TR-107 as a limited run of 10,000 units before going out of business, although far fewer than 10,000 were likely ever built. The company was permitted to call this device a "tricorder" because Gene Roddenberry's contract included a clause allowing any company able to create functioning technology to use the name.
In February 2007, researchers from Purdue University publicly announced their portable (briefcase-sized) DESI-based mass spectrometer, the Mini-10,  which can be used to analyze compounds in ambient conditions without prior sample preparation. This was also announced as a "tricorder". 
In March 2008, British biotech company QuantuMDx was founded to develop the world's first handheld DNA lab, a molecular diagnostic point-of-care device which will provide disease diagnosis in under 15 minutes. In March 2014, the company launched a crowdfunding campaign to support clinical trials of the device and to name it.  Contributors and members of the public called for the device to be officially named a "Tricorder".
In May 2008, researchers from Georgia Tech publicly announced  their portable hand-held multi-spectral imaging device, which aids in the detection of the severity of an injury under the skin, including the presence of pressure ulcers, regardless of lighting conditions or skin pigmentation. The day after the announcement, technology websites including Inside Tech  and The Future of Things  began comparing this device to the Star Trek tricorder.
On May 10, 2011, the X Prize Foundation partnered with Qualcomm Incorporated to announce the Tricorder X Prize, a $10 million incentive to develop a mobile device that can diagnose patients as well as or better than a panel of board-certified physicians.  On Jan 12, 2012, the contest was officially opened at the 2012 Consumer Electronics Show in Las Vegas.   Early entrants to the competition include two Silicon Valley startups,  Scanadu and Senstore, which began work on the medical tricorder in early 2011.
On Aug 23, 2011, moonblink's tricorder app for Android was served with a copyright infringement notice by lawyers for CBS and it was deleted from the Android Market by Google. On January 5, 2012, it was put back again as a new app in the Android market, although it is no longer available. 
In 2012, cognitive science researcher Dr. Peter Jansen announced having developed a handheld mobile computing device modeled after the design of the tricorder. 
Toys and replicas
The first mass-produced tricorder replica was featured in AMT's Star Trek Exploration Set in 1974, followed shortly thereafter by a palm-sized version in Remco's 1975 Star Trek Utility Belt which was sized and marketed to young children in hopes of taking advantage of Star Trek: The Animated Series that was on TV at that time. 
The first life-sized tricorder was produced by Mego Corporation in 1976 and was actually a cassette tape player made to look like a tricorder.  In the 1990s, Star Trek replicas were mass-produced by Playmates, Playing Mantis, and Master Replicas, making commercially produced replicas affordable to the average fan for the first time.