As a rule, the slower the film, the better quality picture you will get.  ISO 100 film often is capable of providing sharp pictures with good color and find "grain" (sort of a sandy, granular texture that shows thru in the picture).  Fast films (ISO 400 or higher) often produce pictures that have reduced sharpness, poorer color and more "grain".  You should always use the slowest film that suits your needs.  For example, if taking pictures outdoors on a sunny day, don't use ISO 400 or higher as you won't need the extra speed of that film and the results will be poorer. 

You'll want to use high-speed films (ISO 400 or higher) when taking pictures with low light levels and you won't be using a flash.  These high-speed films may also come in handy when shooting fast moving objects or when you are on a moving object (boat, etc.).  This will allow you to use a fast-enough shutter speed so camera or subject movement won't be very noticeable.

Making the decision on WHICH film to use is an important one as you will sacrifice quality for speed.

Types of Film
There are two basic types of film:  negative and positive.  Negative film is used to product negatives from which prints can easily be made.  Positive film is used to produce slides. You can't easily view a negative as they are a "negative" image and have an orange mask that's used in the printing process.  But, they are relatively easy and inexpensive to print.  Slides on the other hand are "positives".  You can view the slide easily by either projecting it or, better yet, viewing it on a light-box with a loupe. 

Lightbox

Loupe

Negative film rarely produces the kind of sharp photographs with good color rendition that you may expect.  Although the negative may be very sharp and have all of the color information to produce a great picture, the printing process is typically automated and a machine has to interpret what colors are actually in the negative.  The printing process is VERY susceptible to error and rarely is a negative printed properly. To get a good print, you'd have to have it printed by a master printer (e.g., an experienced human) - and the cost would be very high.   Positive films can produce great results without making prints because you can view it directly without printing it.  Viewing slides with a loupe with a light box will show you all of the sharpness, contrast and color that you really have in a way that you could never see in an ordinary print.  If you decide to print a slide, however, the cost is often higher than printing negatives.

Color, Color, Color, B&W, Infrared or Digital
Film can "see" light.  But different films see different light. If you want to shoot color film, other than the question of positive or negative, you have to decide WHICH film to shoot.  Kodak, Fuji, etc.  Let's say you choose Fuji.  Then you have to decide whether to shoot Provia, Sensia, etc.  There are LOTS of choices when it comes to color film and every color film has different characteristics.  Some films have very saturated colors (good for landscapes, bad for people).  Other films have very realistic soft colors - good for people, bad for landscapes.  Yet other films have high or low contrast. You should get used to ONE film, use it until you get to know its characteristics.  How it records early morning light, what the colors will look like on a cloudy day, etc. 

There are fewer choices when it comes to B&W film as fewer people use it.  The main difference in B&W films is the grain, contrast and tonal values.  The developing process also has a lot to do with the quality of B&W.

Infrared film actually "sees" infrared light - something that is outside the range of the human eye.  So the pictures come out very different than with conventional film.  You never really know what you're going to get until you see your pictures as you can't see what the film will.  Something fun to experiment with, but not very commonly used.  For more information on Infrared Photography, see Spearmint's Guide to Infrared Photography.

"Digital Film" isn't film at all, but just a place to store bits and bytes.  It's more like a disk drive than it is film.  There are many similarities in the way a digital camera records an image and the way film does.  There are also many differences.  But, like film, once you get used to how a digital camera records different kinds of light, it will seem very similar.