Himself Just the facts (and maybe a few opinions of my own) Cedric Diggory is...was...a sixth-year student at Hogwarts School of Witchcraft and Wizardry, of about age seventeen.  He was in Hufflepuff House, and appropriately so, for as Dumbledore said he exemplified many of the qualities of that house.  He was loyal, kind, and brave, almost to a fault.  He obviously valued fair play, as he helped Harry throughout the Triwizard Tournament a number of times even though they were rivals.  He was the captain and Seeker of the Hufflepuff Quidditch team, and also the prefect for Hufflepuff House.  Cedric was arguably the most popular boy at Hogwarts, and certainly one of the best students; he probably would have had a chance to become Head Boy if he had lived that long.  Heck, he might have even been the heir to Hufflepuff for all we knew, but now we'll never really find out.  He was extremely handsome, and it seems every time Harry saw him walking down the halls he was surrounded by a gaggle of female admirers (including Fleur Delacour, the beautiful part-veela girl from Beauxbatons).  But out of the many girls Cedric could have his pick of, he chose Cho Chang, the Seeker of the Ravenclaw team and, unfortunately, Harry's crush as well.
Which leads me to the interesting like/dislike relationship with Cedric and Harry.  In the third book, Prisoner of Azkaban, Cedric made his first (albeit brief) appearance when Hufflepuff's Quidditch team played against Gryffindor's in the first match of the year (Cho was also mentioned briefly in the third book when Gryffindor played Ravenclaw).  Cedric's team ended up winning, something the Gryffindors have never quite forgiven him for. ^^;;; This was a foreshadowing of what was to come in the fourth book, where that one Quidditch match was played out on a larger scale, the Triwizard Tournament.  Again, Harry and Cedric were rivals, but this time they were helping each other along the way.
The first instance of this was Cedric's comment as he and Harry left the Great Hall after their names had both been picked from the Goblet of Fire.  "So, we're playing against each other again."  And although the Hufflepuffs thought of Harry's being picked as another Hogwarts Champion as a threat to their champion's glory (and the Slytherins thought of it as another excuse to put down Harry with those "POTTER STINKS" badges), Cedric didn't feel that way about it.  He didn't seem to resent Harry for also being picked as a champion... sure, he was a little skeptical when Harry said he hadn't put his name in the Goblet, but who wouldn't be?  But he certainly didn't hate Harry for it, as his fellow Hufflepuffs seemed to.  Shortly after the four champions had been chosen, people with "Support Cedric Diggory" badges seemed to be everywhere.
Then Harry found out from Hagrid about the dragons, and when he realized that Madame Maxime and Karkaroff had told Fleur and Krum, the other champions, about them as well, he decided to tell Cedric.  I'm sure if Cedric had found out about them and Harry was the only one who hadn't, he'd do the same.  At first Cedric was suspicious, as Harry was when Cedric gave him the hint about the second task, perhaps thinking that Harry was just making it up so that Cedric wouldn't be prepared for the real task.  But he was still grateful to Harry for letting him know, and because of this he asked the Hufflepuffs to stop putting Harry down and to take off those ridiculous badges that said "POTTER STINKS" whenever you pressed on them.
For the second task, it was Cedric who helped Harry.  Apparently Moody told Cedric to open the egg underwater, because it was this way that the mermaid's song could be heard.  Cedric figured he owed Harry one for the dragons, so what did he do?  Tell Harry to go take a bath and take the egg with him.  Okay, so it wasn't as direct or as helpful as Harry's hint about the dragons, and at first Harry thought Cedric was joking.  When he finally did decide to try it out, Harry managed to figure it out on his own, which was fortunate.
Part of the reason that Harry took Cedric's help with a grain of salt was because at the time he was very jealous of him.  The reason?  Cho, of course.  Harry asked Cho to the Yule Ball, but it turns out she had already been asked by Cedric.  Harry ended up having to go with Parvati Patil, whom he had no real interest in, and whenever he saw Cho and Ced together he had "the strong desire to kick something". ^-^ (and if Harry could have his way the "something" would end up being Cedric himself, but that wouldn't be wise as Ced could (and might, just out of self-defense) kick him back, harder ^-^)
During the third task, Harry and Cedric helped each other out a few times.  Harry heard Krum using the Cruciatus Curse on Cedric (Krum being under the influence of the Imperius Curse himself) and came to his aid (as any decent human being would have done).  When they were both just feet away from the Cup (Cedric was closer, though) Harry saved Cedric from the spider and they both defeated it.  They both kept insisting that the other one take the cup, until Harry came up with the idea that they both take it, and it'd still be a Hogwarts victory.  Cedric agreed, and that's what they did.  If Cedric had said something like "I'm getting that Cup if it's the last thing I do," he wouldn't be too far from what actually happened.
Turns out the Cup was a Portkey - Moody/Crouch Jr. had bewitched it - and they were both transported to the graveyard near the Riddle House where Voldemort and Peter Pettigrew lived.  Voldie (I've always wanted to call him that), upon seeing that two people had been transported, ordered Pettigrew to "Kill the spare", and Peter used Avada Kedavra.  And thus, Cedric was killed.  He was honored in a speech by Dumbledore at the Leaving Feast back at Hogwarts.  Mr. and Mrs. Diggory didn't blame Harry for the incident (although I suppose Fudge does, because he doesn't beleive that Voldemort has risen), and they wouldn't accept Harry's prize money when he offered it to them.  No one at Hogwarts (especially Harry, the other champions, or Cho) are likely to forget Cedric after what happened, and his death signaled the beginning of a new war against Voldemort as he attempts to rise to power once more.