Isekai

is a genre of Japanese manga, anime, light novel, video game, etc. It has character(s) who go into a different world. 
The types of Isekai.
There are mainly two ways for the character(s) to go to a different world. 
One is . In some stories, the character goes to the different world keeping their memory. But there are also stories that the character remembers their previous life after being grown up. 
The other is . The way of moving to a different world is different between stories. For example, the character may suddenly move to a different world, or be made to move to a different world by somebody like god. 
Examples.
Anime that are regarded as Isekai include "", "That Time I Got Reincarnated as a Slime, Overlord, No Game No Life, Log Horizon," etc.
There is some debate as to whether some anime such as "Sword Art Online." In this anime, the protagonist goes into a virtual world and eventually get out.