NoPa stands for North of Panhandle. As you guessed, it is north of Golden Gate Park's panhandle. There is a neighborhood association called NOPNA, which stands for "North of Panhandle Neighborhood Association" (creative, huh!). The NOPNA website contains a lot of happenings in the neighborhood.
The "NoPa" neighborhood name came into usage in 2000 after a push by the neighborhood association, NOPNA, and an article in the Best of the Bay edition of the San Francisco Bay Guardian by Mat Honan.
The neighborhood sometimes referred to as Alamo Square is often considered "NoPa."
The area is served by the NOPNA neighborhood association.