If you're totally clueless about a film, your best option is to just shoot a test roll and see what happens. If the film's expired, consider shooting it a little faster than otherwise (a stop or two), depending on how well the seller says it was stored. If you don't have enough rolls for one of them to be a test, just shoot it anyway, and whatever happens, happens. After all, you're using film in the digital age, so just enjoy it.some film rolls
How to pick a film
This information is accurate to my experience as of the year 2014. Things may have been different in the past, and will certainly be different in the future. But as of right now, here are some basics about selecting which film to shoot with.
Color Negative Film
This is the most commonly available film. Drugstores still sell this in the camera section, and any camera store that sells film will have color negative film. There are differences between each color film, but in general, exposure latitude will be large (much larger than digital), grain will be visible before lens sharpness matters, you don't have to worry about color balance (or what color the light source is), and everything will just work.
Black and White Film
These films are also common, but you'd have to go to a camera store to find them. Online this stuff is extremely easy to get. Nearly all b&w films still available are pan-chromatic, meaning they're receptive to all colors of light (this was not always the case). Ilford makes only b&w films, Kodak has the "classic" Tri-X, among several other b&ws in their portfolio, and Fujifilm makes Neopan. All b&ws tend to have enormous exposure latitudes, meaning the rated ISO is more of a suggestion than a rule.
This is the old film choice of "professionals". When exposing slide film, you shoot a "positive" and not a negative, the exposure latitude is narrow (about the same as digital sensors, sort of), the light source matters and is clearly evident in the photos, and sharpness and image detail are the greatest of any film. Very few slide films are still being made � only Fujifilm still makes any � although you can find old stocks available second-hand. Slide film is still used in large formats by fine art landscape photographers. In the small formats photographers prefer it for the color vividness and control.the stash
Just as there's much ado about digital cameras' sensor sizes, film sizes are equally important. The smallest size still commonly available is 110, which is mostly in the Lomo market. Still available in the second-hand market, APS film is the next biggest, although it was never widely adopted.
And then there's 35mm film. 35mm film was so common that is has become the metric against which all digital cameras are measured. Lens focal lengths are given in "35mm equivalent" and sensor sizes are described in their relation to the size of a 35mm film frame. 35mm film was ubiquitous before digital, as the convenience of loading the film from canisters, the number of frames available on a roll (24 or 36 most commonly), the low price, and the flexibility in camera style choice made this pre-eminent film for family snapshots, news reporters, sports photographers, travelers, and nearly everyone else, too.
But before 35mm was king, there was medium format. Sharing many characteristics with 35mm rolls, medium format comes on rolls, is (kind of) easy to load, and has a huge number of camera models supporting it. Not to say that medium format ever went away � the medium format community remains strong, as the huge frame size allows certain photographic techniques that are difficult (or expensive) to replicate digitally. Medium format rolls are 6cm tall, and cameras shoot frames most commonly at 6x4.5cm, 6x6cm, 6x7cm, 6x9cm, and 6x17cm. Each successively larger frame size gives more and more detail to the photo, but means less and less photos per film roll. Also, a camera that shoots a big 6x9cm frame is large and heavy, especially if there's a mirrorbox involved.
The biggest, though, is large format. Large format starts at 4x5in sheets of film, and only gets bigger. There's no real cap to the size, but why would anyone shoot film this huge? Because of the detail!