Crossworlds fantasy is a subgenre of fantasy which typically involves traveling between an ordinary world and a magical one. It is very similar to portal fantasy - indeed, the line between the two subgenres often becomes blurred, with the main distinction that portal fantasy, unlike crossworlds, can also include time travel.
The main protagonist of crossworlds fantasy usually comes from the ordinary part of the story setting - typically, our thoroughly mundane world - and is transplanted, by supernatural means of some kind, to the magical world, in which the main story arc takes place. Thus in Chronicles of Narnia, for example, the protagonists go through a wardrobe (The Lion, The Witch and The Wardrobe), through a painting (The Voyage of The Dawn Treader), etc. In my own fantasy trilogy, Quest of the Messenger, the magical substance of Stormstone is used for the construction of gates between the worlds.
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The protagonists of the crossworlds subgenre can range from a very clear Chosen One trope (I.e., there is an obvious reason why this person and not any other was taken to the magical world), to a totally ordinary person who just happens to be there. There may be hereditary or acquired qualities that make such transition possible - age being one of them as, for instance, Peter and Susan are told at the end of Prince Caspian that they will return to Narnia no more because they are too old.
Either way, the world travelers will usually find themselves in the midst of some very cool stuff, whether it's learning magic, influencing global politics or fighting an evil sorcerer. By the end of the story - and sometimes a lot sooner - it becomes clear that the main protagonist has played a key role in the salvation, advancement or change of the world to which they had been transferred.
As the main protagonist is typically cut off from home entirely, it is natural that thoughts of returning occupy them a lot, sometimes more than anything else, like it happens for Dorothy in Wizard of Oz, whose every effort is dictated by the longing to go back to her aunt and uncle. The Pevensie children in Narnia, on the other hand, do not seem very preoccupied with going back to their families.
