Starters usually contain a fairly large number of items. In some cases these items are randomly assorted, while in others they are pre-set, and occasionally some combination of the two. In almost all cases, they are designed to provide a set of cards or figurines that is sufficient for a new player to play a game according to the rules -- if not necessarily with a high probability of winning.
In general, starters include rulebooks for their games. Often, other accessories are included as well -- whatever is necessary to provide a new player all of the minimum requisites to play.
Starter packs are generally priced to serve as good small gifts, with prices comparable to a compact disc, a hardcover book, a new t-shirt, and other such items.
Starter packs are complimented by their smaller, cheaper counterparts -- booster packs.