The place is a little tricky -- there isn't a menu, just some pictures of things, and they don't speak much English (if at all.) It helps to know some Spanish, or what you want.
Luckily, the pupusas (basically thick corn tortillas stuffed with ingredients) are pretty easy to order -- as far as I can tell, they have cheese, pork, or bean options. I got one of each, and it comes with Salvadoran cole slaw, kind of chopped cabbage with a red sauce. All of that was only $4.50, though I'd recommend another one or two if you want a full meal. Still, pretty darn cheap.
They're freshly made right in front of you, and they were really good. I liked them all, though the pork and bean ones were my favorite, but I'd definitely get them all again. And if you put the cole slaw on top, even better. It's certainly not fancy food, but it's good and cheap.
Another important fact -- it's more of a takeout joint. There are a couple small tables, but not really a good place to eat.
Unfortunately I don't know much else about the place, as there isn't any sort of sign that I saw, but it's a good spot. Here's more on them from Yelp.