Economy class tickets are the cheapest tickets on a plane, offering the most basic services. Airlines typically divide planes flying local flights within a country into economy and business class seats (international flights often have a third division into first class seats), with the business class seats at the front of the plane (so business class passengers can disembark first), and a curtain seperating the economy class seats and the business class seats. Business class passengers are usually fewer passengers to the toilet than economy class passengers, have better cuttlery and better meals on board.
Economy class passengers may be allowed less luggage on board than business class (typically 20kg checked-in luggage, but Emirates have upped this to 30kg), and aren't allowed access to the business class lounge. When checking in, economy class passengers have longer queues that they need to make their way through. Low cost carriers usually only have economy class seats.
The legroom in economy class is significantly less than in business class, and on long-distance flights conditions may feel like what it must be like for a battery hen - minimal room for manoevre, given water, food and hardly any natural light.
Air France refers to its economy class seats as "Voyageur".