A low-cost airline is generally an airline operating aircrafts
with only on class, usually economy. Also low-cost airlines offer
lower fares than legacy, or holiday carriers hence the name
low-cost. Most low-cost airlines operate fleets mostly consisting
of Boeing 737's or Airbus A320 series aircrafts.
Some examples of low-cost carriers would be:
WestJet-Canada
SouthWest-USA
JetBlue-USA
RyanAir-UK
EasyJet-Europe