"God decrees that there are twelve (lunar) months- ordained in God’s Book on the day when He created the heavens and earth; four months of which are sacred..." - Surah At Tawbah (9:36)
The Islamic year is a lunar year - based on the phases of the moon; this is unlike the Gregorian year which is a solar year. The lunar year is approximately 10 days shorter than the solar year - this is why Ramadan moves back 10 days in the solar year, every year. There is great wisdom in this! This allows Ramadan to occur in different seasons over time. If not for this variation, some parts of the world would always have Ramadan in long, summer days and other parts would always enjoy short fasting days in cooler months.
The Islamic year starts at the year of hijrah (migration) - when the divine decree came instructing the nascent Muslim community to migrate from Makkah to Madinah.
There are 12 months in the Islamic calendar, 4 of these are sacred months in which fighting is forbidden. These are: Rajab and then the three consecutive months of Dhul Q'ada, Dhul Hijjah, and Muharram.