Such directories contain an alphabetical listing of persons, their occupation, often where they worked, and their street residence. Usually family members are not listed therein, unless they are of late teen or adult ages, and those people may then be students or working.
But the directories also have lists of churches, their pastors or priests, and their locations, including when they were founded. They contain lists of all kinds of social and fraternal organization along with the officers. Ward boundaries are given, as are street locations. There may be advertisements with pictures of someone's business in the advertising sections.
But what can make them especially useful besides placing someone for a census return, is that often there will be entries for where someone dies - which can precede when official vital records exist. They can be a list of deaths (a mortuary list, as its often called); or there may be interlineated death dates along with someone's name, showing that Mr. So and So died on such a date. Also useful are the directories that show when a woman got married (Ms. X married Mr. A, on such and such a date), and that people moved out of the area. You won't waste your time looking in Massachusetts for someone who moved to Colorado. Wive's names are often shown in parentheses after the husband's name.
I am familiar with a local project where a volunteer has gone through 16 years of directories which predate local official civil records, and has north of 20,000 entries showing just the information described above. Wouldn't you like to know that your person in town A moved to town B after marrying Mr. X?
So how are these arranged? Generally the names appear alphabetically, and the directories were published annually. In our own local case, the first and second ones are seven years apart because there simply wasn't that much of a change. The third is four years later, then three, and gradually they began to be published annually. These are especially important when you are looking for someone in an unindexed census, since if they lived at the same address they before, the year of, and the year after the census, there's a good likelihood that they will be easier to find. If they moved every year, then you will have an advance clue of their footloose tendencies. If the directory indicates renters, homeowners and the like, this could clue you in to the possibility of property records if they owned the house and lived there when they passed away.
How can you find them? Some are online on commercial sites. Others are on film at the Family History Centers of the LDS church. Most are in libraries and historical societies, and may be in book form or on microfilm or microfiche. They tended to be done on cheap paper, so often the older directories are in very bad shape.
What is being done, and is there easier access? My own library was fortunate enough to get a multiyear grant and will be digitizing a hundred years of city directories (and possibly 150, depending on copyright negotiations), and we will make these available for free on the web. The technology is there, so ask your local librarians and historians what their plans are, or ask those in the areas that you need to research.