What are pagers? Why are they still being used to communicate by people?

Hezbollah is in shock after their fighters’ communication devices, the pagers, became targets of coordinated mini-explosions, resulting in deaths of 9 people with over 2700 others injured. In the statement, Hezbollah claimed that the pagers used by the members working for the group started exploding mysteriously at around 3.30 pm (1230 GMT).