Vampires
In the vampire diaries, vampires must feed on blood to survive. They have the ability to compel humans, their stronger than humans, they can run very fast, they have heightened senses (especially hearing), they can jump very high for example from the ground to the roof of a building, but they cannot walk in the sun unless they have a "daylight ring." In the show vampires are also portrayed to have the ability to "cut off" their humanity switch, which means they can "cut off" their feelings/emotions.
Vampires are immortal but can be killed with a wood stake in the heart. Vervain also weakens vampires.
Stefan Salvatore
Stefan Salvatore became a vampire in 1864, he was shot and had vampire blood in his system, so he became a vampire. His brother is Damon Salvatore. Stefan is portrayed as a genuine, down to earth vampire. Stefan is a "ripper" at his core which means he can't control himself when he feeds on humans and in the beginning of the show and in some points of the show Stefan is portrayed as a vampire on an "animal" diet. Stefan is also one of the main characters in the show. In season 1 Stefan had already been a vampire for more than 100 years so throughout the show there are "flashback" scenes that focus on his origin story and what it was like before he became and vampire and right after he turned.
Damon Salvatore
Damon Salvatore is another one of the main characters. He is the more rebellious brother and doesn't really care for human life until he starts forming a life in Mystic Falls. Damon turned into a vampire at the same time as his brother in 1864 and they both were shot. When Damon first showed up in Mystic Falls, he had his "humanity switch" off which showed the true nature of vampires.
Elena Gilbert
Elena is the most important main character out of all of them. The show is honestly based around her and what her life is like and then as it goes on it adds in all these other characters storylines. In the first couple of seasons Elena was not a vampire but eventually she dies with vampire blood in her system and turns into a vampire. But the reason Stefan and Damon two very old vampires were interested in her and wanted to know her is because she looked just like Katherine Pierce (Katerina Petrova). They all later come to find out that Elena and Katherine are each other's doppelganger. Elena was also not a vampire for as long as the others she eventually took the cure.
Caroline Forbes
Caroline got turned pretty early in the show, probably around season 2. When Caroline got turned, she had no idea what was happening to her, she didn't even know about vampires however all of her friends did know. She eventually figures out how to deal with being a vampire.
Enzo St. John
Enzo doesn't really come into vampire diaries until the later seasons. But he is an older vampire and knew Damon from centuries before the time period in the show. He was portrayed as a very loyal and caring friend. But he also had little to no care for his actions. Eventually he ends up dating Bonnie Bennet and was very romantic and protective of her but when Stefan had his humanity off, he killed Enzo trying to get to the cure.