Islamic calendar is lunar (based on the phases of the moon). It starts with Hijra - or 16 July, 622 CE, the day when Muhammed left Mecca to present-day Medina. 
The 12 months are:

Muharram ("forbidden") – During this month, Gabriel brought the first revelation of what would become the Quran. Traditionally, it was unlawful to fight during this month.

Safar ("void")

Rabi' al-Awwal ("the first spring").

Rabi' ath-Thani ("the second spring").

Jumada al-Ula ("the first month of parched land").

Jumada ath-Thaniya ("the second month of parched land").

Rajab ("respect" or "honor") – A sacred month. Fighting was traditionally forbidden.

Sha'ban ("scattered") – The time when Arab tribes scattered to find water.

Ramadan ("scorched") – The most venerated month, during which Muslims fast.

Shawwal ("raised") – Traditionally, camels begin to rise after giving birth.

Dhu al-Qa'da ("the truce") – Another month during which war was banned.

Dhu al-Hijja ("the pilgrimage") – Traditionally, the hajj to Mecca was a spring pilgrimage.

Tags: Islamic months, Religion