Dimmensions are different realms. In each dimmension you can have its own version of a world, such as Agrabah, Draculania and Nowhere. Sometimes, such as the case ofBlossom and Blossom Z their are changes in their appearances. Dimmensions are not usally connected, however it is known that if you kill a younger counterpart to a hero, and your from another dimmension it will cause your dimmensions version of him to die as well, but slowly and painfully
In the center of the dimmensions are the core worlds, worlds that are mega worlds of a certain demistration ( A nick mega world Eg) that keep the worlds in other dimmensions with those characters alive. Other authors like Omnitrix1 and Sacred keybearer 66 exist in their own dimmensions, but it is possible to traverse the dimmenisons.
Some examples of dimmension differences