Jonathan Kent, married to Martha Kent, is the adoptive father of Clark Kent, who is better known as Superman. Before his recent death, he was a rural farmer in Kansas. He is frequently referred to as Pa Kent within the comics. Jonathan Kent and his wife Martha found Kal-El's rocket after it crashed on earth and took him in as their own son. They raised their son to have strong moral values and to hide his abilities from the world for fear that he would be taken away and experimented on like a lab rat. The first time Jonathan Kent died when Superman/Clark died after his battle with Doomsday, Jonathan also died in the Brainiac story arc and helped to guide his son back to the world of the living.
Johnathan Kent was created by Jerry Siegel and Joe Shuster for use in DC Comics. His first appearance came in Superman #1 which was released in 1939.
Along with his wife Martha, Johnathan was extremely instrumental in regards to instilling good values in a young Clark Kent, and molding him into the (super) man he eventually becomes. Encouraging Clark to use his many gifts for the betterment of mankind, you could say that Johnathan and Martha also deserve credit (though they would never ask or accept it) for each and every life Superman saves.
Jonathan Kent in other media
Jonathan Kent has had a long and varied history in other media- in fact, his history is almost as extensive as the Man of Steel himself. Apart from some minor one-season absences in short lived series, Jonathan has been in the majority of Superman's major film and television endeavors with the exception of his first- the Fleischer brothers 1941 animated Superman serials.
The Adventures of Superman (Novel): 1942
Pa Kent is referred to as Eben Kent in the first novel length Superman story to see print. He is a hardworking farmer who is down on his luck, and enters an anvil lifting contest at the county fair to win the desperately needed prize money to save his farm. Unfortunately, the exertion proves too much for him, and he dies soon afterward. On his death bed, he instructs Clark to use his powers for the good of mankind.
It is in the Kirk Alyn Columbia serials where Jonathan Kent makes his first appearance, played by Ed Cassidy. Interestingly enough, he is not called Jonathan Kent in this series, but rather Eben Kent. He would continue being called Eben throughout the 1952 George Reeves television series.
On another note- while characters like Lois, Jimmy, Perry and others have undergone drastic changes throughout various film and television incarnations, (apart from his name) the core of Jonathan's character has remained fairly stable- he almost always appears as the the honest, midwestern-value oriented farmer that instills in Clark a sense of duty, honesty, responsibility that forms the foundation of his superhero credo.
The Adventures of Superman (TV series): 1952
Tom Fadden plays Eben Kent in this series.
Superman (TV series) 1975:
George Chandler plays Jonathan Kent in this short-lived TV series.
In the most well-known of Superman's cinematic endeavors Glenn Ford plays Jonathan Kent. While his role in this film (and relatively non-existent role in the subsequent films, outside of flashbacks) might
have been minimal, his impact on character and development is substantial. Case in point? "Son, you are here for a reason... and it is not to score touchdowns..." In fact, towards the end of this film where Clark must listen to either Jonathan or Jor-El, he remembers Jonathan's words instead.
Superman (TV series): 1988
In another short-lived series Alan Oppenheimer plays Clark's earth-bound father.
Superboy (TV series): 1990 - 1992
In this lesser-known proto-Smallville Superman title Stuart Whitman plays Jonathan Kent and, due to the nature of the show (focusing on Clark as a young 'un) Jonathan has a more active role than in many previous films and TV series.
Lois and Clark: The New Adventures of Superman (TV series): 1993-1997
In perhaps the best-known 90s Superman series Eddie Jones plays
Jonathan Kent. Eddie's Jonathan is the most enduring and likable Jonathan since Glenn Ford many years earlier. Though older, heavier and retired, Jones manages to keep the charm of Jonathan's home-spun rhetoric alive.
Superman (animated TV series): 1996 - 2000
In the ultimate animated Superman offering of the 90s Mike Farrell plays Jonathan Kent. In this animated series we see Jonathan in the Eddie Jones style- older, retired, but still Jonathan. Mike Farrell would continue to play Jonathan in the Justice League (2004) and Superman: Brainiac Attacks (2006) animated offerings.
Smallville (TV series): 2001 - 2011
In the most recent and long-running Superman franchise we find a Jonathan that we have not seen in many years. John Schneider plays a younger, more involved Jonathan than his recent incarnations (which were the older, retired versions). In Smallville, Jonathan is more than a side note offering a helpful one-liner. He is an active part of the adventure, often being effected by Clark's mishaps.
It should also be noted that this is an updated Jonathan Kent, one who struggles to live in a modern world. However, he does so with the same grit, determination and unshakable moral strength that he has exhibited for decades. Watching Schneider in this series it becomes easier to see why Jonathan Kent is such an interesting and important character- he helps form the basis of Clark's moral fiber- which is one of the most impressive things about the Superman mythos as a whole.
Man of Steel: 2013
Kevin Costner will be playing the role of Johnathan Kent in the upcoming Superman reboot, directed by Zack Snyder. Like his comic-book counter part, Johnathan Kent strives to instill proper values in his adopted son, Clark.
Young Justice (Animated TV Series) 2011-
In season 2 (Invasion) Johnathan has apparently grown close to the Super-Family, calling Superboy his "boy".