Preservation Hall is literally little more than a hole in the wall--walk by too fast, and you'll miss it.
But if you do happen to catch the sign over the tired old doorway, pay the tiny cover charge and make it into the small, crowded, un-air-conditioned, lit-by-a-single-bare-bulb room to hear whoever is fronting the Preservation Hall Jazz Band on that particular night, you are guaranteed to get a sample of authentic New Orleans jazz.
Nowadays, much of the French Quarter is geared towards the college (and post-college) folk, attempting to make it into spring break year-round. As result, much of the music you hear booming from many of the bars and clubs on Bourbon Street is an Allman Brothers cover band or, worse yet, bumping and grinding techno.
Where can you go to get a little true jazz? Preservation Hall. It don't serve no drinks, there ain't many seats, there's barely even any light--but the music that's played in the tiny room is by far superior to anything within a 6-block radius. This is Old Jazz as Louis played it. The band plays a set, then takes a break, and then plays another set, and once you're in, you're free to stay as long as you like.
While you're there, feel free to make a request--it's a dollar for a jazz standard, and five for them to play "When the Saints Go Marching in," presumably because they're tired of playing it.
Just go. It's jazz without frills, and it's more than worth the price of admission.