The Flash is a superhero in the DC comics family of characters. Four men have taken on the title of the Flash at one time or another.
The first Flash was Jay Garrick. A lab accident involving fumes from hard water gave him his powers of super speed. He sported a red and blue costume with a large yellow lightning bolt extending diagonally across his chest and a round, metal hat. He lived on Earth-2 of the DC multiverse and was a member of the Justice Society of America. Later on, he was shown coexisting with other Flash incarnations on Earth-1.
The second Flash was Barry Allen. Similarly to Jay, Barry received his powers from a lab accident - this time involving a lightning bolt that struck a rack of chemicals in the forensic lab at which he worked. An extra-dimensional energy called "The Speed Force" began to emanate from Barry's existence which acted as the source of power for all heroes and villains that utilized super speed. He manifested an increased metabolism, total control over his molecular structure, and the same superhuman speed that Jay had. Inspired by the older hero, he took on the name of 'The Flash' and acted as protector of Central City, Missouri. He became a founding member of Earth-1's Justice League alongside Superman, Batman, Wonder Woman, Green Lantern, Aquaman, and the Martian Manhunter. He dated and later married Iris West who worked as a reporter in Central City.
One day, Iris brought her nephew named Wally to visit with Barry. The boy was a huge fan of the Flash and openly wished that he could have superpowers like him. Fate just so had it that Barry's lab was set up the exact same way on the night of the accident and a stray lightning bolt came through the window just as it did before, striking a rack of chemicals and granting Wally super speed powers. Barry gave him a costume and took him on as a sidekick named 'Kid Flash'. Together, they shared many adventures until tragedy drove the two apart following Iris' death at the hands of Barry's nemesis: Professor Zoom.
Barry disappeared for a time, traveling to the future where he reunited with Iris (who was revealed to have been born there in the 30th century). Soon, a crisis demanded Barry's return to the present day where he battled the threat of the Anti-Monitor who waged war on the DC multiverse. Choosing to sacrifice himself for the benefit of hundreds of worlds, Barry entered the Speed Force and remained there for many years - presumed dead by his friends and family back on Earth. In his absence, Wally decided to take on the mantle and become the third Flash, protecting Keystone and Central cities from the villains that plagued them both.
At first, Wally's decision to act as the Flash was met with criticism from the public. They accused him of trying to replace Barry. However, in time, Wally earned their trust and admiration - fully stepping into the role and growing in his power by way of the Speed Force. He became a member of the JLA and developed a close bond with his teammates. Similarly to Jay and Barry who shared friendships with the Green Lanterns of their times (Alan Scott and Hal Jordan respectively), Wally developed a solid friendship with Kyle Rayner who was the Green Lantern of the JLA during Wally's tenure as the Flash. The two had a rocky start to their relationship but overcame it and remained friends for a long time.
After some time, Iris West reemerged from the future and approached Wally with a concern. She and Barry had managed to have kids who in turn had children of their own who manifested the same super speed powers that their grandfather possessed. One of her grandchildren - Bart Allen - struggled to control the powers that he was born with. He aged at a more rapid rate than normal and was likely to die of old age before his teenage years. Wally trained him to control his speed and brought him down to a sort of equilibrium for a while. The boy took on the name 'Impulse' and acted as a hero for some time under that moniker. Later in his life, he became the second Kid Flash and eventually took on the mantle as the fourth Flash, albeit for a brief period. His evil clone named Inertia gathered a group of villains in a plot to steal Bart's power for his own use. Bart succeeded in stopping them, but at the cost of his own life. His death ended his time in the suit of the Scarlet Speedster.
The role of the Flash bounced back and forth between the four heroes through DC's history and have made a number of appearances across TV and film. Jay Garrick recently appeared as the Flash in the Young Justice cartoon as well as Batman: the Brave and the Bold and the DC Universe Online video game.
Barry was depicted as a member of the Justice League in the Super Friends cartoon series as well as recently on Young Justice, The Batman, and his own television series entitled 'The Flash' which ran for one season. In the near future, Barry is set to appear on the CW's television series 'Arrow' which is expected to spin off into a separate series starring the character. Barry has appeared in a number of films as well. Justice League: Doom, Justice League: New Frontier, Justice League: The Flashpoint Paradox, and a Justice League television movie starred Barry in the role of the Flash. His video game appearances include DC Universe Online, Mortal Kombat vs. DC, Injustice: Gods Among Us, and the handheld game based on the TV series.
Wally has appeared in a number of television and movie capacities. The Justice League and Justice League Unlimited cartoon series utilized the character with some elements of Barry Allen interwoven into the backstory. He appeared as Kid Flash on Batman the Brave and the Bold and recently Young Justice. He has had one movie appearance in Justice League: Crisis on Two Earths (implied but not explicitly stated). He has starred in a number of video games including one based off of the Justice League cartoon series and a standalone one entitled 'Justice League Heroes' which drew inspiration from the cartoon. He is set to appear in a video game based off of the Young Justice series in the near future.
Bart Allen appeared as Impulse on the Smallville television series and recently in Young Justice. He was originally slated to appear in the Justice League cartoon alongside Robin and a female version of Cyborg but did not make the final cut. Instead, a Wally West/Barry Allen amalgamation was included. He appears as Kid Flash in the DC Universe Online video game.