A felony is any criminal offense where the maximum possible sentence is some number of years in state prison. Felonies can be initially charged in a District Court, but cannot remain there until the end of the case. Not unless it is a felony which, by statute, is capable of being disposed of by a sentence to county jail. In the case of those types of crimes it is up to the prosecution to decide whether to leave it in District Court -- and thereby waive the chance to seek state prison time -- or to indict it and move the case to Superior Court.
The following are some typical offenses that cannot be prosecuted to a conclusion in District Court.
The following are offenses that are felonies, but which have the option of being prosecuted in District Court if the prosecution is content to seek only county jail time as punishment:
Kidnapping (Some kinds, anyway)