Not too simplistic, yet not overly complex, this article will summarize all isekai terminology for you.
1. Who is Truck-kun, known by every isekai fan?

A common trope in manga is the main character often dying in traffic accidents. They are frequently hit by a truck, thus reincarnating into a new world. In this world, they may retain memories of their past, find a reason to continue existing, and be equipped with a new body for adventure.
Due to the overuse of the truck trope, anime fans have even given the truck a name, 'Truck-kun,' as a 'force' in isekai. It's even considered a character rather than just a plot detail, adding a humorous twist to isekai scenarios.

One of the oldest examples of the 'Truck-kun' story is Astro Boy. This character was created when an inventor, Astro's father, lost his son in an accident, and he was created based on the image of that son.
Minky Momo, another classic anime character, also ran into trouble when she was hit by a toy truck causing an accident. Minky was then reincarnated when she became the biological daughter of an Earth couple who adopted her. It can be said that the truck is an indispensable plot device when transitioning a main character from the ordinary world to the fantastical realm.
3. What is the true meaning of isekai?

In Japanese, isekai is the reading of the characters '異世界', which can be translated as another world or a different universe. In the world of manga & anime, isekai stories occur when the main character ventures into a new world. Although isekai has only gained prominence in the past decade or so, many anime fans believe that isekai has existed for decades, only going unnoticed.
4. Where does Isekai draw inspiration from?

Isekai is somewhat inspired by a tale from the land of cherry blossoms. The character Urashima Taro had an adventure diving into the ocean, visiting a new world, and befriending a beautiful princess. However, the story's ending isn't very happy as when Taro returns to shore, hundreds of years have passed and there's no one he knows left alive.
Alongside, Western works have also left their mark on isekai, including Alice in Wonderland and The Wizard of Oz. Both of these series have even been adapted into anime.
5. How is Isekai categorized?

The isekai genre is divided into 2 branches: 'isekai ten’i' & 'isekai tensei'. While the former involves reincarnation, the latter involves being transported to a new world without dying. Many previous anime works fall into the isekai tensei category, such as Fushigi Yûgi, Magic Knight Rayearth, Inuyasha, and the famous film Spirited Away. In modern isekai, Sword Art Online is considered a pioneer in using virtual worlds as a new frontier for humanity.