The Islamic calendar is a lunar calendar where the Gregorian is a solar.
A lunar calendar is a calendar which is based on the lunar cycle.
There are 354.37 days in a lunar year. Islamic calendar has a drift about 10-11 days with the solar calendar and months are not linked with the season because the cycle is 355 days where solar cycle is of 365 days.
It comes back to the position it had in relation to the solar year approximately every 33 lunar years.
The lunar calendar is used mainly for religious purposes, and in Saudi Arabia it is the official calendar.