Hel is an area within Niffleheim where the dead go when they are neither honored nor dishonored. It is located on the lower section of the World Tree Yggdrasill. It is ruled by the powerful being Hela. It is unknown if the realm is named after her or vice versa.
Bruce Banner was killed by Loki and sent to Hel where he was given an ideal life with Betty Ross and a child. Thor and Loki went to Hel to merge the souls of Bruce and the Hulk, who Loki and Amora had separated.
Loki tricked Hela and she brought Hulk to Hel, thus saving Odin. Hulk began destroying the realm until he was merged with Bruce. Hela sent Bruce back to Midgard. She then used the souls there to trap Loki in amber. She promised that when Thor died he would go there, though Thor continues to fight her on that.