Banh mi literally refers to crispy, French-inspired baguettes with the Vietnamese substitution of some rice flour for wheat flour which lends a lighter texture to the bread. More commonly, however, banh mi are extremely cheap and tasty sandwiches sold throughout Asian areas. Back in the day, these sandwiches were often referred to as "dollar subs" for obvious reasons, although certain types now cost up to three dollars.
A typical banh mi consists of three key elements: bread, sauce, and various fillings. The bread, as previously mentioned, is a large (approximately 6" to 8", but possibly larger depending on where you go) baguette-style bun. These buns are often sold very cheaply by themselves at Asian markets, although making Vietnamese subs at home is rather pointless in my opinion (not only is it time-consuming, but this may be one of the rare cases where it is actually cheaper to purchase the sandwich from a vendor).
The second element, the sauce, is highly variable among banh mi vendors, although it is generally agreed to consist of both mayonnaise and butter. This is often accompanied by a generous smearing of pâté and possibly a drizzling of fish sauce. If you haven't figured it out by now, Vietnamese subs are far more forgiving on the wallet than they are on the arteries.
The fillings tend to vary, although there are certain standard ingredients. These include pickled daikon, carrot, cucumber, sliced pork roll and ham, topped off with a few springs of cilantro. Technically, this version is a banh mi thit, but as it is the accepted standard that last part normally tends to be omitted. Some places offer banh mi made with beef or chicken (not so quite common now thanks to bird flu), or even vegetarian versions involving bean curd or gluten. This combination of ingredients makes for an immensely satisfying meal, with the tangy, crispy vegetables meshing well with the light sweetness of the bread and sauce and the delectable smoothness of the fatty meats.
As with anything so cheap there are certain drawbacks. Often, you may come across unappealing chunks of translucent fat from the meat floating randomly around the sub, or bread which is just a bit too crispy. Like many things from Chinatown, they often seem to be haphazardly thrown together. Beware of the "spicy sub" option, as the spice is often very unevenly distributed. Don't undertake to remedy this situation yourself by squirting a packet of hot sauce all over the sub--when you do get to the spicy part it will be hot enough to take your head off. But on the other hand, at this price one can probably afford to be fairly heavy-handed about these things.