Economically speaking, an oligopoly occurs when a market is dominated by a few large firms. These firms each have enough market share that all firms in the market can be affected by each others' pricing decisions.
The impact on a consumer depends on whether the firms in the market compete or whether they collude and form a cartel. A cartel is only possible in an oligopoly. Since there are so few firms, they can conceivably agree on prices that will give them a high level of profit (rather than competing fiercely). If they do so, the consumer is harmed.
If firms in oligopoly compete, then consumers get the benefit of lower prices and improved quality of products. This is true, in part, because of the size of the firms in oligopoly. Large firms that are actually in competition are most likely to engage in innovation (think of the innovation in automobiles, for example) and are also most likely to enjoy economies of scale.
When firms in oligopolies actually compete, this market structure is quite beneficial. When they do not, it is similar to a monopoly in that it causes higher prices and lower quantities produced for consumers to buy.